An optical power monitor comprising a pig-tail fiber and a GRIN lens opposed to each other with a spacing and each having an end face (angled face) angled with respect to the axis, being low on reflection loss and able to be easily assembled. The pig-tail fiber and the GRIN lens are fixed in respective sleeves having axial end faces adhered to each other. At least one of the pig-tail fiber and the GRIN lens has an arc-segmental end face directed in its axial direction at the tip of its angled face. The arc-segmental end face is positioned in the same plane as the axial end face of the sleeve of the one. The axis of the sleeve of the one is kept vertical to the axial end face of the sleeve of the one. It can be prevented that the axial tip of the other enters the sleeve of the one and comes into contact with an inner wall of the sleeve of the one. Therefore, the assembling man-hours needed to adjust their relative positions can be reduced, and the assembling yield can be improved.