A light output monitoring apparatus for monitoring a power of light output from a light emitting end of a catheter incorporating an optical fiber, the apparatus comprising: a light receiving unit for receiving light output from the light emitting end of the catheter; an attachment unit provided at a position opposite to the light receiving unit;an adapter (20, 20A) detachably attached to the attachment unit, fixed in position by being attached to the attachment unit, and including a through-hole; a protective cap (30, 30A) including an insertion opening provided at a first end and into which a part of the catheter of a predetermined range on the light emitting end side is removably inserted, including a window portion provided at a second end and for transmitting the light output from the light emitting end of the catheter, removably inserted into the through-hole of the adapter (20, 20A) from the second end side, and fixed in position by being inserted into the through-hole, wherein in a state where the adapter (20, 20A) is fixed in position by being attached to the attachment unit, the part of the catheter of the predetermined range on the light emitting end side is inserted into the insertion opening of the protective cap (30, 30A), and the protective cap (30, 30A) is fixed in position by being inserted into the through-hole of the adapter (20, 20A), an output light power is monitored by causing the light output from the light emitting end of the catheter to pass through the window portion of the protective cap and to be incident on the light receiving unit, and wherein the protective cap (30, 30A) includes a hole, and a hole cross-sectional area increases toward the end near the first end. a light receiving unit for receiving light output from the light emitting end of the catheter; an attachment unit provided at a position opposite to the light receiving unit;an adapter (20, 20A) detachably attached to the attachment unit, fixed in position by being attached to the attachment unit, and including a through-hole; a protective cap (30, 30A) including an insertion opening provided at a first end and into which a part of the catheter of a predetermined range on the light emitting end side is removably inserted, including a window portion provided at a second end and for transmitting the light output from the light emitting end of the catheter, removably inserted into the through-hole of the adapter (20, 20A) from the second end side, and fixed in position by being inserted into the through-hole, wherein in a state where the adapter (20, 20A) is fixed in position by being attached to the attachment unit, the part of the catheter of the predetermined range on the light emitting end side is inserted into the insertion opening of the protective cap (30, 30A), and the protective cap (30, 30A) is fixed in position by being inserted into the through-hole of the adapter (20, 20A), an output light power is monitored by causing the light output from the light emitting end of the catheter to pass through the window portion of the protective cap and to be incident on the light receiving unit, and wherein the protective cap (30, 30A) includes a hole, and a hole cross-sectional area increases toward the end near the first end.