Consumed energy is minimized to reduce running cost. Final formed products of uniform quality are successively obtained. High productivity is provided. A primary forming step of obtaining a primary formed product by plastically working a heated steel sheet with a primary forming die set for a predetermined time period, a secondary forming step of machining the primary formed product with the secondary forming die set for a predetermined time period to obtain a secondary formed product, and a hardening step of allowing a cooling die set placed in a hydraulic press controlled by a servomotor to hold the secondary formed product under pressure for a longer time period than the time period of each of the primary and secondary forming steps to obtain a hardened final formed product are successively performed in sequence. The hardening step is performed independently without coordinating with the primary and secondary forming steps.