1. A simulation device which simulates motion of a robot system comprising a plurality of robots each comprising a drive shaft driven by a motor, the simulation device comprising: a simulation unit which executes, by simulation, a motion program for each robot in which each robot is set in advance so as not to interfere with one another, the motion program containing a command velocity and a command acceleration of the drive shaft, and a motion waiting instruction; a motor load calculation unit which calculates a motor load acting on the motor at every predetermined time based on a simulation result obtained by the simulation unit; a storage unit which stores in time series a line number of the motion program and the command velocity and the command acceleration at the line number associated with one another and further stores in time series the motor load further calculated by the motor load calculation unit associated with the line number; a robot load calculation unit which calculates a robot load acting on the robot based on the motor load stored in the storage unit; an execution time calculation unit which calculates an execution time of the motion program for each of the line numbers stored in the storage unit based on a simulation result obtained by the simulation unit; a waiting time calculation unit which calculates a motion waiting time in accordance with the motion waiting instruction based on the execution time calculated by the execution time calculation unit; a load designation unit which designates an allowable range of the motor load; a waiting time designation unit which designates an allowable range of the motion waiting time of the robot; and a program modification unit which modifies the command velocity and the command acceleration such that the motor load stored in the storage unit is in the allowable range of the motor load designated by the load designation unit and that the motion waiting time calculated by the waiting time calculation unit is in the allowable range of motion waiting time designated by the waiting time designation unit.