A fast gate module has been developed at KEKB, which can pick up the signal of one bunch from a train of bunches. The gate module is attached to a turn-by-turn beam position monitor, where the beam phase is detected by an orthogonal phase technique. Characteristics of the gate module have been investigated. We have measured the bunch phase along a long train followed by a gap. The phase shift in the train was compared with a calculation based on the transient beam loading. The longitudinal displacement of the collision point was estimated from this phase shift.