If you are planning to spend more than 3 days in Japan, you can consider purchasing a Japan prepaid data SIM card (either 1GB data or unlimited data with speed restriction, that is valid for 14 days) for 4,310 yen (approximately S$50) instead of paying about S$18-22 daily to the local telcos (using StarHub as an example) for unlimited data usage in Japan.
The following is a step-by-step guide on how to complete an online purchase of a Japan prepaid SIM card (data only) from Singapore. And collect it at the airport upon arrival.
1) Check that your mobile device is compatible under Supported Devices.
2) Visit b-mobile VISITOR SIM. And select size of SIM card and where you would to collect/receive it. I usually opt for 1GB prepaid (type) (which allows me to make fairly clear Skype calls from my mobile phone) and to collect it at Airport post office.
4) Complete the purchase form. As you will need to present your passport (as a form of identification) during collection, remember to input your full name found in your passport (under the Name field).
7) If you have opted for Airport post office, simply present your Order number and passport at the Airport post office to collect your package.
8) After inserting your SIM card, you may need to configure your mobile phone’s APN setting before you can start using it.
If you have any questions, do post it as a comment below and I will try to respond to it.