The light power monitoring device includes: a first optical fiber; a second optical fiber connected to the first optical fiber; a low-refractive-index resin layer which covers (i) a connection between the first and the second optical fibers and (ii) a predetermined region of the second optical fiber which region extends from the connection toward a forward-propagating-light-output side; a high-refractive-index resin layer which covers a region of the second optical fiber which region is not covered by the low-refractive-index resin layer; and an outputted light detecting device which is provided at a position corresponding to an end of the low-refractive-index resin layer which end is located on the forward-propagating-light-output side of the second optical fiber or at a position which is away toward the forward-propagating-light-output side of the second optical fiber from the position corresponding to the end of the low-refractive-index resin layer.