In a wavelength monitor that monitors a wavelength of laser light emitted from at least two semiconductor lasers formed in parallel on a semiconductor substrate, the wavelength monitor includes a collimating lens that collimates laser light from each of the semiconductor lasers, an etalon that is arranged so that laser light collimated by the collimating lens is capable of entering and has a periodicity, and a photodetector that receives laser light transmitted through the etalon and detects a light intensity, wherein a beam propagation angle in the etalon of laser light emitted from each of the semiconductor lasers becomes a predetermined angle obtained by formula 1.