If you need emergency medical help in Japan, call 119 for an ambulance and seek immediate emergency care. The Japan Visitor Hotline, 050-3816-2787, offers visitor assistance 24 hours a day; it is not ambulance dispatch. JNTO: emergency numbers and visitor hotline
For a planned visit, the useful first step is to confirm how the facility accepts patients and what reception needs from you. This guide explains that process; it does not select a medical department, test or treatment for you.