I want to use WhatsApp on Samsung GT S5830 in Japan, but I do not have a SIM card for my phone. Can I use the Internet on my mobile via WiFi?
You can: during the registration process, insert a phone number of your property.
Wait until whatsapp tells you to manually input the code you should have received via SMS at the number you had selected.
Once you have completed these steps, you will be able to use your whatsapp account from your phone using wifi connection.
If you couldn't verify your number, follow the official guide provided by WhatsApp.
If you're told that it can't connect to your wi-fi, click here to see the related article on the WhatsApp FAQ.