The present invention relates to an optical power monitor (1) having a reduced number of component parts, easy to assemble, capable of facilitating positioning for achieving improved optical coupling, and having a reduced size and a reduced manufacturing cost. A pigtail fiber (2) in which the center axis (5a) of an optical fiber is shifted from the center axis (4a) of the columnar capillary (4) by 0.020 to 0.150 mm is used. The pigtail fiber (2) and a photo diode (3) are provided in a cylindrical tube with the pigtail fiber (2) center axis and the photo diode (3) center axis aligned with each other. The pigtail fiber (2) and the photo diode (3) can be adjusted to the optimum position in the cylindrical tube by adjustment in the longitudinal direction, thus advantageously simplifying the adjustment process in comparison with that in the conventional optical power monitor in which adjustments by movement in the radial direction, movement in the longitudinal direction and rotation about the center axis are made.