It is an object to provide an optical component holding unit whereby the positioning accuracy can be improved while reducing cost through facilitated assembling and ensuring reproducibility. The fitting between a cross-sectionally noncircular-shaped rectangular convex portion 8a and a rectangular hole and the fitting between a cross-sectionally circular-shaped circular convex portion 11a and a circular hole, which surround or form opening portions 22a and 22b for allowing passage of light, guide adjacent optical component holding units 35 to predetermined positions and couples the units detachably to each other for positioning in the three-dimensional direction, while the fitting between the rectangular convex portion 8a and the rectangular hole and the fitting between spring pins 13a and fitting holes restrain the adjacent optical component holding units 35 relatively in the rotation direction for positioning in the rotation direction.