To check your data usage page, please follow the directions below:

After logging into your "My Sakura Mobile" account, please go to the "Manage Plan" page
For information on how to get to your "Manage Plan" page, visit here.

1) Select the contract you would like to view its record (Refer to ① in the images below)

2) Select "Contract details" (Refer to ② in the images below) 

3) You can view your remaining data as well as your daily usage and call history (Refer to ③ in the images below)