A device for measuring the reflection factor by irradiating a measurement area of a microchip with light, and in which a light receiving part is made to receive light reflected from the measurement area for determination of the reflection factor of the measurement area. The light receiving part is located in an angular region .theta., satisfying the relationship (1/2).alpha..ltoreq..theta..ltoreq.sin.sup.-1(1/n) and being located between angles .theta..sub.min and .theta..sub.max, .theta..sub.min and .theta..sub.max (in.degree.) being angles which the reflection light forms with respect to a normal on the edge of the irradiated surface of the area to be measured in a virtual plane which contains the emission center of the light emitting part and which is perpendicular to the microchip, wherein .theta..sub.min is 1/2 .alpha. and .theta..sub.max corresponds to sin.sup.-1(1/n), where .alpha. (.degree.) is the scattering angle of the light radiated by the light emitting part which is located directly above the area to be measured, and wherein n is the index of refraction of the transparent component.