I live in Japan, there are many very good doctors/clinics in what seems like small crappy, old buildings - and the average one doctor-run clinic is small and rents/space is expensive so I would not judge the "building" too much in this particular case.
They don't have hundreds of patients per day so costs need to be kept in check and in Asia, real estate cost in large major cities is a big factor.
I just wanted to add information. I think his clinic is run by him and his assistants.