To provide a detector in which a labor required for setting the execution timing of a shift function is eliminated.

With a reference state where the negative pressure in a pressure inlet pipe, communicating with a sucker is stabilized and suction of a work, is prepared, the pressure is brought to zero and the variation value is set as a threshold Pth. Furthermore, a time elapsing before the referential state is brought about after a shift signal is inputted from a PLC, i.e. a time To, elapsing before an intermediate time point for actually sucking the work is reached after the referential state is brought about, is set automatically. Upon receiving the shift signal from the PLC, a timer incorporated in a pressure sensor is actuated, and a detected pressure value is shifted, when the shift timer value To measured by the incorporated timer elapses. In other words, when the shift time To elapses, the work suction system detects suction, by comparing the threshold value Pth, i.e. a variation value from the referential state, with the detected pressure converted into the variation value from the reference state.