An electric-component mounting system wherein a determining device is operated for presently required operations of working devices disposed on a common main body, to determine a cycle-time-determinant working device which determines a cycle time of the system such that the cycle time is determined by a required operating time of the cycle-time-determinant working device as calculated when the presently required operation is performed at predetermined maximum acceleration and deceleration values or a predetermined maximum operating speed, and a slowdown device determines actual acceleration and deceleration values or an actual operating speed of each of non-cycle-time-determinant working device, so as to be lower than the predetermined maximum acceleration and deceleration values or operating speed thereof, such that the presently required operation of each non-cycle-time-determinant working device can be completed within the cycle time determined by the cycle-time-determinant working device.