To manufacture a small read head by arranging a reflection detector receiving a ray from a reagent pad and forming an optical path completely preventing arrival of a single reflected ray substantially.

A housing, which is provided with an upper wall 36, a lover wall 38, a side wall 40, a slanting wall 42, a flat back wall 44, and a front wall parallel to the back wall 44, is arranged in a read head 34. A bulb 46 of a radiation light source is supported right above a reagent pad 30 by means of the housing 48. A light beam from an irradiated area in the reagent pad 30 passes through a detection window 68 provided with slanting edges 69, a detection window 70 provided with slanting edges 71, and a window 72 toward a detection area, in which four reflection detectors 57, 58 and the like are arranged. Inside the read head 34, an irregular shape type baffle plate 74 constructed of the first flat partition wall 76, the second flat partition wall 78, and a zigzag partition wall 80 is arranged. The shape of the baffle plate 74 is designed so as to prevent arrival of a ray, which is reflected once, from the reagent pad 30 to the detection area.