Abstract: A float switch and a control apparatus of an internal combustion engine are disclosed. This float switch comprises a float attached to a main body so as to be vertically swung around one end thereof, a magnetic material fixed to the float, and a contact adapted to be closed or opened in response to the approach or removal of the magnetic material. The float switch is provided in the pathway of a collant passage and a lubricating oil passage of the engine. The control apparatus has a warning apparatus which is operative in response to an absence signal from the float switch which is indicative of the absence of the fluid that is being sensed, and has a rotating speed control apparatus to reduce the engine speed. The absence of the fluid is informed to the operator by the warning and the speed reduction. A warning apparatus for an outboard engine to control the engine speed when an abnormality occurs is also provided.