PURPOSE: To prevent oil leakage while simplifying the manufacture and the assembly by fastening two connection terminals solidly on an oil cap while the connection is made in a detachable insertion design between the connection terminal and the guide shaft and an elastic terminal and the base.

CONSTITUTION: Two connection terminals 8 and 8 is fastened solidly on an oil cap 1 by insert molding and guide shafts 12 and 12 are inserted detachably into the connection terminals 8 and 8 while a float 15 with a electroconductive plate 17 is inserted into the shafts 12 and 12. Elastic terminals 23 and 23 having a bent part 24 are mounted to a base 19 into which the lower end of the shafts 12 and 12 is inserted detachably. As the oil level reaches the lower limit position, the float 15 comes down to the lower end of the shafts 12 and 12 until the electroconductive plate 17 gets in contact with the bent parts 24 and 24 of the elastic terminals 23 and 23 to allow current to flow through the connection terminals 8 and 8 and a detection lamp lights to indicate the running out of oil.