Abstract: A liquid level detecting device which comprises an annular float adapted to float constantly on the surface of a body of liquid subjected to level detection, a barrel passing through the float so as to guide the motion of the float, permanent magnets disposed in the float and a magnetically operatable lead switch disposed in the barrel permits the detection of liquid level to be effected on the principle that the lead switch is magnetically turned into its ON state at the time that the float, while in motion, reaches the position at which the barrel incorporates the lead switch.