What is a good jobs platform to hire senior-level (manager) positions in Japan?

Wantedly is one of the most well known hiring platform for startups in Japan.
There will be a fee for you to post the jobs, but you can post unlimited jobs with fix amount of monthly fee. 
Also if you want to find like a country managers, I would recommend to join some startup related events in Japan. 

There are startup weekend, startup cafe which you can join as audience also you can do pitch in events or become a mentor to connect to the people who are interested in working in startups.