A seat back frame (25) includes vertical longitudinal frames (27, 29) disposed in a left position and a right position to each other, and transverse frames (31, 33) linking the longitudinal frames (27, 29) to each other, a seatbelt retractor (35) is configured to provide an emergency locking, a seatbelt (5) is linked retractably to the seatbelt retractor (35) and provided with an engagement member (23), a locking member (21) fixed to a longitudinal frame (27) and configured to disengageably lock the engagement member (23), an upper part supporter (47) is provided at an upper part (27a) of the longitudinal frame (27) and configured to support the seatbelt (5) from the seatbelt retractor (35) to return, and the seatbelt retractor (35) and the upper part supporter (49) are arranged to transmit a load acting from the seatbelt (5) on the upper part supporter (47), in a compression direction of the longitudinal frame (27).