PURPOSE: To detect the liquid level of an oil tank safely, by piling up plural inverted V-shaped blocks having optical rotators through prescribed gaps in the vertical direction and combining and using a polarizer and an analyzer.

CONSTITUTION: A liquid level detecting device consists of plural inverted V-shaped blocks 31W3n, a tank 1, and a liquid 2 such as oil in the tank 1. Each block 3 consists of an optical rotator 4 and rhombic light-transmitable materials 5 and 6 which hold the optical rotator 4 in both of left and right sides and are made of glass, and blocks 3 are piled up through prescribed gaps G (G1WGn-1) in the vertical deirection. A polarizer 8 polarizes the incident light to one light-transmittable material 5, and an analyzer 9 receives the light from the other light- transmittable material 6. In this constitution, the safety in liquied level detection is secured because the sensor part is not electrically constituted.