To complicate the issue, until recently there was no clear legal impetus for a divorced foreign national to report the divorce to immigration. Some people seemed to just fly under the radar for as long as possible.
Recent updates to the immigration laws have made this issue much clearer:
In short, if you get a divorce, you'll need to apply for a change in status of residence as soon as possible if you intend to remain in Japan.
The procedure at immigration follows a few steps:
First, within 14 days of your divorce, you are obligated to report the divorce to the Immigration Bureau. You can report in person at your nearest regional or branch office, or you can use the bureau's e-Notification System to report the divorce online.
Your status of residence will not be immediately revoked upon notification. A grace period is allowed for divorced individuals to get their affairs in order and arrange an application for a change in status of residence. If you intend to pursue a status of residence change, some possibilities include:
1) Working Visa
If you are working in Japan, you can change to a working visa. This will require demonstrating to the bureau that you have a specialized skillset and will also require the cooperation of your employer. For example, if you have been teaching at a high school as a full-time teacher for a number of years, you can make a strong case for being granted a 教育 status of residence.
2) General Resident Status for Childcare
If you are the primary caregiver of a child following your divorce, you can apply for a 定住者 visa. This is loosely translated as a "resident" visa, and makes no restriction on your type or place of employment, but it is not permanent. The 定住者 visa generally needs to be renewed at one year intervals, and if your primary reason for being in Japan is to raise a child, you will need to continue to demonstrate that you are caring for and raising that child at renewal periods.
3) General Resident Status
If you were married for an extended period (at least three years), and you can demonstrate a continued, stable livelihood in Japan following the divorce, it is still possible to acquire a 定住者 status of residence. Having a child and citing proximity to that child will bear consideration in this case but is not strictly necessary for approval.
If you fail to apply for a change in status of residence, and 6 months elapses from the date of your divorce, the Immigration Bureau may choose to revoke your status of residence at its discretion. Presumably in this case you will be notified of the change (such as a letter sent to your last registered address or a phone call to your last registered number) and asked to leave the country by a certain date.*
*[If any readers have personal experience or further knowledge to share on how such a notification is made, please share in the comments section below.]