An insulation resistance decrease detector for an industrial vehicle is disclosed. The detector includes a coupling capacitor, a sensing resistor, an AC voltage applying section having a first terminal and a second terminal, a voltage detecting section, and a determining section. The AC voltage applying section applies an AC voltage of a predetermined frequency to the vehicle body through the sensing resistor and the coupling capacitor. The voltage detecting section detects the voltage at a node between the sensing resistor and the coupling capacitor. The determining section determines that a resistance value of the insulation resistor has decreased when the level of voltage detected by the voltage detecting section becomes less than or equal to a predetermined level.