A condition monitoring system for a wind turbine generator includes an orientation sensor installed in a nacelle and configured to detect an orientation of the nacelle and a condition monitoring unit configured to receive an output from the orientation sensor and perform at least any one of recording, analysis, or transmission of a signal. The condition monitoring unit preferably detects, based on the output from the orientation sensor, the movement along the rotation direction of the nacelle associated with a torsional oscillation of the tower. The orientation sensor can also detect slow directional fluctuations that are difficult to detect using an acceleration sensor or the like, and thus, can satisfactorily detect the rotation of the nacelle associated with the torsional oscillation of the tower.