Public Wi-Fi for free use have become more and more readily available. Tourists will encounter these networks at airports, major railway stations. Most of Public WiFi require cumbersome advance registrations, short term access on an hourly and limited to specific devices (Smartphone, iPhone).
Paid Wi-Fi hotspots are avaible, A one-day pass typically costs around 500-800 yen and gives you access. You can use all devices not only smartphones but also laptops and some services require a Japanese credit card.
To use prepaid / Rental SIM cards, your device has to meet requirements.
Your device must be UNLOCKED.
The device must support one of the following frequency
4G(LTE) : Band 1 (2100MHz), Band 19 (800MHz), Band 21 (1500MHz)
3G(W-CDMA/HSDPA/HSUPA): Band 1 (2100MHz), Band 19 (800MHz)
In Japan,frequency bands is not the same all places.
Your device does not suppurt freaquency of place, you might not get connected.
If you are not familiar with phone settings, Rental Portable WiFi is one of choice.
Portable (Mobile) wifi can be picked up airport counters , or arrangements can be made for delivery to your accommodation in Japan.
There are 5 major network for Pocket Wi-Fi.
The largest network coverage! Usable in various places, even in subways! docomo's mobile Wi-Fi uses their network, which covers the largest areas in Japan and is stable and consistent. With their network, you can connect with the internet even you are in buses, trains, and subways. Although they don't offer any unlimited data plans, considering using it mainly for SNS or Google Maps, their offerings are just sufficient for your needs.
Unlimited data, recommended for use in urban areas! UQ WIMAX offers a plan, allowing you to use the data up to 3GB in 3 days. It is well connected in urban areas, however, it may have trouble in some rural areas yet. You don't have to worry about it too much if your connection is limited after you over the data capacity. Because their official website announces that even in that situation, they offer the internet speed allowing you to watch YouTube relatively smooth.
Unlimited data, recommended for use in urban areas! Y!mobile offers a service called unlimited data advanced mode, which is available only with devices compatible with it. It enables you to connect with the internet without worrying about the data capacity and the transmission speed. Although the service does not available in many rural areas yet, you can use it in urban areas without any problems. Regular mode covers area wider than that of with the advanced mode, however, if you over the data capacity of 7GB, the transmission speed decreases to 128kbps. Note that if you change the mode to the advanced in that situation, the speed does not be changed until that month is over.
Unlimited data, covering local areas! KDDI is a parent company of WiMAX, and accordingly you can use the 4G LTE by au in the areas covered by WiMAX 2+, which is wider than that of by WiMAX. Also the downloading speed becomes faster accordingly. If you go to local areas, the trouble of unconnected with the internet will be decreased as you can use WiFi via the network by KDDI.
Unlimited data, covering local areas! SoftBank is a parent company of Y!mobile, and they offer the network wider than Y!mobile's. Available with the areas compatible with 4G LTE, and the data communication is available in the areas as wide as ones offered by docomo and au. Some WiFi routers provided by SoftBank offer unlimited data services, and which are said not to be limited on the data much. However, they offer only a few as a rental service for travelers.