In a synchronization judging circuit, a measuring portion measures a signal to interference power ratio of a received signal at each frame. An averaging portion calculates a moving average of a series of signal to interference power ratios for a predetermined number of frames to produce a mean value at each frame. A counter counts a number of consecutive frames each of which the mean value meets a predetermined threshold requirement. The counter further compares a counted value with a predetermined threshold value to decide whether to perform a transmission control operation or not.