PURPOSE: To enable high-accuracy measurement of an inclination angle, by installing light-receiving apparatus and arithmetic operation appararus available for arithmetic operation of a light quantity of a luminous apparatus by 2 light- receiving elements located in an opposed manner to the luminous apparatus by conversion into an electric signal detected and making either of the luminous apparatus and light-receiving apparatus movable corresponding to an inclination angle to be measured.

CONSTITUTION: Simultaneously with the main body 3 as it is inclined through an angle of θ1, a light-receiving block 5 is swung. and the center line of the block 5 creates an angular difference by the angle θ1, which is equal to the inclination angle θ1 relative to the center line of a light source 4 installed in the main body 3. In this condition, the light quantity P1 received by a light- receiving element 6a becomes predominant over that P2 receiver by the light- receiving element 6b. As a photo-diode generating a current in proportion to an incident light quantity is so mounted that is faces the elements 6a, 6b as the source 4, the light quantity difference of 6a, 6b allows determination of the inclination angle θ1 in nearly the same linear same linear relationship with the angle θ1.