Abstract: A movable mechanism (3) moves a first housing (1) having a light emitting element (4) mounted thereon and a second housing (2) having a photo sensing element (5) mounted thereon in a state that those housings are parallel to each other. The movable mechanism (3) includes a pair of knobs (15A and 15B) and a spring (11) urging those knobs to move apart from each other. When the knobs (15A and 15B), while resisting the force of the spring (11), are pressed so as to make a mutual approach of those knobs, the first housing (1) and the second housing (2) are moved apart from each other. At this time, a finger of a patient is placed between the housings, and an operator gradually lessens his force applied to the knobs (15A and 15B) and reduces the force to zero. Then, the finger is held between the first housing (1) and the second housing (2). In this case, the orientations of the axes of the light emitting element (4) and the photo sensing element (5) are variable irrespective of the thickness of the finger.