Rotor behavior may affect the torque characteristics of gerotor pumps, but the measurement of the behavior has received little attention. Thus, in this study, we measure the rotor behavior of a gerotor pump used for automatic transmissions under various operating conditions. The experiments revealed that the inner rotor rotated near the middle of the cover wall and casing sidewall, slightly tilted with respect to them. The outer rotor rotated in the vicinity of the casing sidewall and came closer to the sidewall with decreases in the dimensionless parameter ?N??P (where ? [Pa?s] is the oil viscosity, N [s^(-1) ] is the rotational speed, and ?P [Pa] is the pressure difference). In addition, the eccentricity of the outer rotor was observed to increase with decreases in ?N??P. The inclination of the inner rotor and the approximation of the outer rotor to the casing sidewall may increase the driving torque of the pump. Taking the measured results on rotor behavior into account, a clear physical meaning is given to an existing mathematical model of viscous friction torque.