To provide a ventilation monitoring device for a cab of a construction machine, capable of monitoring the conditions of ventilating the cab of the construction machine with outside air introduced thereinto.

The ventilation monitoring device for the cab of the construction machine includes an outside air introducing system for supplying outside air through an air-conditioner 10 into a cab 9 with a damper 10g selecting inside/outside air, and an inside air circulating system for supplying inside air from the cab 9 through the air-conditioner 10 into the cab 9. It further includes an air flow detector 22 provided in an outside air suction duct 18, an inside air throttle valve 21 provided in an inside air suction duct 16, and a ventilation control device 11 having a storage part previously storing a reference value (a recommended value) and an operation part for outputting a display signal showing that ventilation is not normal and a volume increase of air from the air-conditioner is instructed and for outputting an instruction to the inside air throttle valve to reduce its opening when the volume of the outside air from the air flow detector is smaller than the reference value (the recommended value) stored in the storage part.