A simulator (10) for estimating a life of a speed reducer includes a rotation speed and load calculator (21) for simulating the operation program of a robot (12) and calculating the rotation speed of the robot speed reducers (G1-Gm) and the load exerted on the individual speed reducers; a storage (22) for chronologically correlating the rotation speed and the load and storing the rotation speed and the load; a speed reducer life calculator (23) for calculating the life of the individual speed reducers, based on the rotation speed and the load; an operating ratio setter (24) for setting an operating ratio of the robot; and a speed reducer life estimator (25) for estimating the life of the speed reducers, based on the life of the individual speed reducers and the operating ratio.