It is important to investigate the auditory dynamics of pitch perception of a sweep tone in order to make auditory processing clear. In recent years, the trajectory of pitch perception of a sweep tone has been estimated within the viewpoint of auditory scene analysis. However, there are few studies to quantify ``instantaneous pitch'' in a sweep tone. In this study, instantaneous pitch in the sweep tone was measured by comparing the perceived pitch of a sweep tone with that of a pure tone. The sweep tone used in the experiments had a duration of 4250 ms and a frequency range of 1 oct. As a result, the instantaneous pitch almost matched the pitch corresponding to the actual frequency of the sweep tone at the middle of duration. However, it was higher at the position of 1/4 of the duration, and lower at the position of 3/4 of the duration than the pitch of actual frequency. It seems that the gradient of the perceptual trajectory is gentler than that of the actual frequency. It was suggested that the dynamic property of pitch perception of the sweep tone was affected by forward and backward pitch in the stream.