Japanese Immigration lawyers can be helpful if you have a difficult case or if you need help working your way through the Japanese visa application process or dealing with the Japanese immigration office, but be VERY careful!
I used one and he charged me about $1,000 but his advice was wrong and indirectly cost me about $10,000 in lost income and extra accommodation!!

Check your country's embassy website to see if they have a list of recommended Japanese immigration lawyers or use one that your friends have used and recommend.
In most cases you will not need one.