PURPOSE: To achieve a measurement constantly with a high accuracy, by correcting a difference between electrical signals obtained from a light receiving element under the condition of receiving a scattered light and an external light and under the condition of receiving the external light only, based on an electrical signal obtained therefrom under the condition of only receiving light from a light emitting element.

CONSTITUTION: In a case 22 having a detection window 21 on the bottom thereof, a rotary plate 23 with a projection window 23A, a light receiving window 23B, a mobile light shielding plate 23C and a reference passive reflector 23D is arranged and rotated with a motor 24. In the first condition, a suspension is irradiated with light from a light emitting element 28 from the projection window 23A and the scattered light is received with a light receiving element 29 together with an external light through the light receiving window 23B. In the second condition, only the external light is received with the light receiving element 29 through the light receiving window 23B. In the third condition, the light from the light emitting element 28 is reflected on the reference passive reflector 23D and received with the light receiving element 29. The effect of the external light is removed by correcting the difference between electrical signals obtained from the light receiving element 29 in the first and second conditions based on an electrical signal from the light receiving element 29 obtained in the third condition.