Multiple elevator doors are driven with one servomotor through a multi-degree-of-freedom linkage system. This paper deals with the derivation of the equation of motion of the door mechanical system. The movement of the doors is described by giving various constraint conditions to multiple single-chain and multi-degree-of -freedom rigid body systems. The door motion was simulated based on this formulation. The conclusions are as follows. (1) It is clarified by computer simulation that the change in the constraint condition caused by integration error is extremely small. (2) The method of obtaining the motor driving torque, in which a given movement of the door or the motor is regarded as a constraint condition, is proposed. (3) The conversion of joint torques to motor torque can be obtained by converting these joint torques into a single-degree-of-freedom system which is motor rotation.