A robot system includes: a robot having a main shaft gear attached to a rotary shaft of a drive unit, a first countershaft gear meshing with the main shaft gear, a second countershaft gear meshing with the main shaft gear, and a third countershaft gear meshing with the main shaft gear. A number of teeth of the main shaft gear, a number of teeth of the first countershaft gear, a number of teeth of the second countershaft gear, and a number of teeth of the third countershaft gear are integers having no greatest common divisor other than 1. As first processing, a first number of rotations, which is a number of rotations of the main shaft gear, is derived based on a phase of the first countershaft gear and a phase of the second countershaft gear, and a second number of rotations, which is a number of rotations of the main shaft gear, is derived based on the phase of the second countershaft gear and a phase of the third countershaft gear, and the drive unit is stopped when the first number of rotations and the second number of rotations do not coincide with each other.