(content edited for brevity and privacy)
There are several issues I see here:
1. Illegal Dimissal
Did they have reasonable grounds to dismiss you without the required notice?
Unless you were grossly incompetent or did something like punch a coworker in the face, they have a legal obligation to honor the contract.
Unfortunely, the burden of seeking justice falls upon the shoulders of the wronged party.
If you were unfairly dismissed, there are basically two main ways to deal with it:
a) You can go to court. However, we wouldn’t recommend this as it is a very time consuming process for what would only amount to you receiving one or two months of salary if you won, offset by the cost of having to hire a lawyer to represent you (paid upfront).
b) Had you been a member at the time, the union could have negotiated on your behalf and potentially arrived at a mutually acceptable resolution. Unfortunately, we can not do this as you were not a member at the time of the firing.
2. 30 Days Notice
In rare and extreme circumstances, a company can get permission from a government agency to dismiss you immediately without notice.
In most cases, a company is required to give you 30 days notice of dismissal, or payment in lieu of that notice.
I recommend that you write to the company requesting the payment in lieu, and if they do not pay up you can then report them to the Labor Standards Inspection Office (LSO).
It can take between two to six months to receive payment this way.
However, even if you were to pursue this route, a company might still not pay what they owe you even if they’re ordered to do.
It can then become a battle to get the LSO to do their job – and it may even require you going to small claims court.
3. Illegal Employment
You said that you were hired illegally, but I can’t see anything outlining why you believe that to be the case.
The only thing I can think of is that they may have had you illegally working on the wrong type of visa (i.e. working on an instructor status instead of a humanities status, or working on a tourist visa).
If that was the case, I would be careful about reporting them as you would be just as guilty of working illegally for them as they were guilty of hiring you to work them illegally, and this might cause issues with immigration later down the line.
Good luck, and if you’re staying in Japan we recommend you join a union as soon as you start your new job.
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