To surely prevent the demounting of an oil level sensor mounted on an oil tank.

A sensor storage recessed part 12 is formed in a state of being inwardly recessed from a side face of the oil tank 10, and a fixing part 14 with an engagement receiving part 16 is formed on an opening part of the sensor storage recessed part 12. The oil level sensor 17 has a constitution that a detecting part 18 is formed on one face of a base part 19 abutted on the circumference of the opening part, a connector part 24 is formed on the other face of the base part 19 for externally fitting and connecting a mating connector 30, and further an engagement arm part 28 having an engagement part 29 positioned outside at a predetermined interval with respect to the connector part 24, is mounted on the other face of the base part 19. The oil level sensor 17 is mounted on the oil tank 10, and then the mating connector 30 is mounted on the connector part 24 to control the inward movement of the engagement arm part 28 by the mating connector 30.