A blow-by gas passage abnormality detecting system for an internal combustion engine detects, only under the condition that the running state of the engine is idle, a leakage of a blow-by gas coming from a blow-by gas passage for returning the blow-by gas produced in the engine to an intake passage, on the basis of a parameter varying with a change in the air flow to be sucked into the engine. The abnormality such as leakage or clogging in the blow-by gas passage is detected if the ISC opening of an idle speed control valve measured as a load amount at an idle time of the engine is smaller than a predetermined value. Alternatively, the abnormality may be detected by detecting the blow-by gas pressure in the blow-by gas passage downstream of a PCV valve by a gas pressure sensor.