PURPOSE: To enable measurement with high accuracy with an extremely simple construction and operation by forming a light transmission body of an arced boundary wall which is so selected as to reflect the light made incident thereto at a max. degree under a given condition and comparing the measurement light and reference light irrespectively of the intensity of a light source.

CONSTITUTION: When a liquid 1 to be measured is contained in a dent 19 and a support 16 is closed, the boundary wall 3 of a light transmission body 2 is brought into contact with the liquid 1. When the fibers 4, 8 are irradiated by lighted from light source 15, the light made incident to the fiber 8 arrives directly as reference light 9 at a photodetection chamber 11. The light made incident to the fiber 4 arrives at the wall 3 of the body 2 and reflects at a max. degree by having the attenuation corresponding to the concn. of the liquid 1. The reflected light passes the fiber 5 and arrives as measurement light 14 to a photodetection chamber 12 and therefore a difference in brightness airses between the reference light 9 and the measurement light 4 projected on the member 13 made of ground glass. Then, the atteuator 10 is operated so as to the brightness of the both projecting light on the menber 13 assume same values. The value, such as concn., corresponding to the manipulated variable is readably applied to an attenuation device 10, by which the concn. or the like of the liquid 1 is immediately known.