An oil level gauge device is configured in such a manner that an oil level gauge is inserted into a guide pipe which has one end provided with an opening and the other end connecting to an oil pan. A cap is provided to the base end of the oil level gauge, and the cap comprises: a plug which is engaged with the inner peripheral surface of the one end of the guide pipe; and a flange which is formed integrally with the portion of the plug which is located on the side opposite the oil pan, the flange coming into contact with the one end of the guide pipe. The inner peripheral surface has formed therein a groove which extends from the inner end side of the plug to the surface of the one end which comes into contact with the flange. When the plug is engaged with the inner peripheral surface and the flange is not in contact with the one end, the groove forms an air flow passage between the plug and the guide pipe, the air flow passage connecting the inside and outside of the oil pan through the guide pipe.