signal is not delivered in a state where all the joint portions are coupled the automobile can be driven normally thus even when a load is applied to the engine and the blowby gas is generated the blowby gas thus generated is sucked by the negative pressure of the intake duct 12 and feed back and hence prevented from being exhausted in the atmosphere when performing the maintenance of the automobile the joint portions 13 15 16 are sometimes disengaged in this respect when these joint portions 13 15 16 are disengaged and then restored after the maintenance if one of these joint portions is carelessly forgotten to be coupled there arises a fear that the blowby gas may leak from the joint portion thus remained in a disengagement state when the blowby gas is generated however since the connector portion at the joint portion thus disengaged is detached the switch circuit c 1 is opened and so the portion between the detection terminals 81 a 81 b of the detection circuit 80 is placed in an insulated state in this state when the ignition key is turned on to activate the detection circuit 80 the alarm signal is outputted from the detection circuit 80 thus a driver notices that one of the joint portions in the blowby gas feedback path is in a disengagement state accordingly the blowby.