A light output monitoring apparatus includes a light receiving unit 11, an attachment unit 12, an adapter 20, and a protective cap 30, and monitors a power of light output from a light emitting end 40a of a catheter 40 incorporating an optical fiber 41. The protective cap 30 includes an insertion opening into which a part of the catheter 40 of a predetermined range on the light emitting end side is removably inserted, includes a window portion 32 for transmitting the light output from the light emitting end of the catheter 40, and is fixed in position by being inserted into a through-hole of the adapter 20. The adapter 20 is fixed in position by being attached to the attachment unit 12. Thus, a light output monitoring apparatus and the like capable of easily performing light output monitoring while maintaining a cleanliness of a catheter are realized.
