The authors carried out an experimental investigation about the torque variation in a single-stage Stoeckicht planetary gear (Type 2K-H) constructed with spur gears with the following results : (i) The torque variation in the planetary gear is caused by the errors per mesh and the run-out errors of each gear. (ii) When the number of error cycles per mesh coincides with all kinds of natural frequencies of the planetary gear, the torque variation of higher-speed shaft increases. (iii) A resonance is caused in the gear testing machine with a planetary gear by the run-out error of the sun gear. Consequently, the torque variation of lower-speed shaft increases.