"FM-AM tones" are defined as fluctuation tones whose frequency and amplitude are periodically modulated by an identical modulation waveform simultaneously. "Principal pitch" is defined as somewhat steady and average pitch perceived from FM sounds with pitch fluctuation. In the present study the principal pitch shift of FM-AM tones was measured as a function of the modulation rate by the psychoacoustical experiment using a method of adjustment. The principal pitch shift interval from in-phase condition to anti-phase condition is nearly constant at the modulation rate of 4 to 7Hz, and becomes larger as the modulation rate becomes faster at the modulation rate of 7 to 20Hz. As the experimental results are considered to reflect the temporal resolving power of the auditory system for frequency modulation, the following hypothesis is proposed: The auditory system is able to follow the periodic frequency modulation when the modulation rate is slower than 7Hz. However, it becomes hard to follow the faster frequency modulation for the auditory system. Furthermore when the modulation rate becomes too fast to follow the modulation process, the perceived extent of FM becomes smaller, and is shifted toward the frequency region accompanied with larger amplitude.