To prevent the occurrence of a troublesome state when a trap elements does not close an outlet properly.

A condenate trap comprises a trap element 12 arranged in a trap chamber 6; and a sensor chamber 16 having a sensor element 24 to monitor performance of the trap element 12. The sensor chamber 16 is communicated with a trap chamber 6 through a first passage 26 and one or a plurality or second passages 28. The first passage 26 is communicated with the sensor chamber in the upper region of the sensor chamber 16 and meanwhile, the second passage 28 is communicated with the sensor chamber in a lower region. Thereby, the sensor element 24 is positioned in a level above that of the lowermost part of a port communicated with the sensor chamber 16. When the trap element 12 does not close an outlet 10 properly, condensate stored in the sensor chamber 16 flows in the trap chamber 6 through the second passage 28. The condensate level is lowered and the sensor element 24 is exposed, the sensor element 24 reacts to transmit an alarm signal.