A light output monitoring apparatus includes a light receiving unit, an attachment unit, an adapter, and a protective cap, and monitors a power of light output from a light emitting end of a catheter incorporating an optical fiber. The protective cap includes an insertion opening into which a part of the catheter of a predetermined range on the light emitting end side is removably inserted, includes a window portion for transmitting the light output from the light emitting end of the catheter, and is fixed in position by being inserted into a through-hole of the adapter. The adapter is fixed in position by being attached to the attachment unit.
