An optical power monitoring device detects an optical power propagating through an optical fiber including at least a core and a cladding. The optical power monitoring device includes: a first optical fiber; a second optical fiber having a larger core diameter than the first optical fiber; a connection part where an end surface of the first optical fiber and an end surface of the second optical fiber are spliced; a first leakage part for leakage of a beam from the first optical fiber to the outside; a second leakage part for leakage of a beam from the second optical fiber to the outside; a first photodetector that detects an optical power leaking from the first leakage part; and a second photodetector that detects an optical power leaking from the second leakage part.