PURPOSE:To provide a non-contacting type liquid level detecting device which has a simple constitution having no mobile section and can detect the surface level of a liquid in a container without being affected by the contamination, etc., of the liquid and container. CONSTITUTION:The light emitted from a light emitting element 3 goes straight ahead, passes through a container 1 having a light transmitting property, and is received by means of a light receiving element 4 while the level of a liquid 2 contained in the container 1 is low. When the level of the liquid 2 becomes higher than a reference value, the light from the element 3 does not reach the element 4, but totally reflected by the surface of the wall of the container, at the boundary between the surface of the light and the wall of the container 1, because the straight line connecting the elements 3 and 4 to each other intersect the surface of the wall of the container 1 at an angle theta.