A power-off door closing device of fire proof rolling door in which the rolling door slides down to close by its own weight under abnormal power-off condition by brake-releasing action or by brake-applied action of a braking pin actuated by a energized solenoid, through successive movement of the first end of a sliding lever of a solenoid with a bent arm, a clamp member and a brake actuating rod. This device can be easily changed into non-failsafe door closing fire proof type with original configurations wherein the solenoid is reverse located so that the brake-released action caused by an energized solenoid or the brake-applied action under abnormal power-off condition applies. A reserved power supply temporarily energizes the solenoid to maintain a temporary brake-releasing under abnormal power-off condition, to enable the rolling door sliding down to close by its own weight.