However, usually you will see this split into family name and first/given/christian name. If this is an on-line thing, it is best to have two text fields rather than one This will remove any confusion later on if people enter their family name in usual Japanese style or if they use the western style by putting their "christian" or given name first and family name last.
If you are expecting Japanese input, you should also have a second set of input fields for the kana readings.
PLEASE if you are expecting a mix of Japanese and western names you allow all types of input and don't put stupid rules that give people with western names grief when trying to enter in.