To establish your company in Japan, you need to register it at Legal Affairs Bureau or Houmu Kyoku (ほうむきょく・法務局).
In Japan, under current rule, only attorney or Bengo-shi (べんごし・弁護士) and solicitor or Shiho-shoshi (しほうしょし・司法書士) can be on behalf of you.
There is a qualification called Administrative scrivener or Gyosei-shoshi(ぎょうせいしょし・行政書士）, but they can’t handle company registration process.
In most cases, to get business manager visa, making your own company is convenient way to show that you have invested your money.
Among Administrative scriveners, if they pass an exam for handling visa process, they can act as an immigration lawyer.
However, even though they are called as immigration lawyer, their qualification is just an Administrative scrivener or Gyosei-shoshi(ぎょうせいしょし・行政書士), so, even if they can handle business manager visa as an immigration lawyer, they are not allowed to handle company registration.
Therefore, if they handle it and act as your agent, it is illegal.
To avoid such a trouble, you should use Solicitor and Immigration Lawyer like me.
I have both qualifications, so, when it comes to applying for business manager visa, I am the right person to help you.