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; and a main shaft phase output unit outputting a phase of the main shaft gear as a first main shaft phase. A phase of the main shaft gear is derived as a second main shaft phase, based on a phase of the first countershaft gear, a phase of the second countershaft gear, and a phase of the third countershaft gear. Processing to stop the drive unit is performed when the first main shaft phase and the second main shaft phase do not coincide with each other.