A laser diode module includes a laser diode device, a first photo detector part for receiving, first divided light that is a first portion of light when at least one of light emissions of the laser diode device is divided into two portions of light each traveling a different optical path, a second photo detector part for receiving second divided light that is a second portion of light thus divided via at least a wavelength selective member, and a controller which controls the lasing wavelength of the laser diode device on the basis of outputs of the first and second photo detector parts. A beam splitter provided between the laser diode and the wavelength selective member divides light emitted from the laser diode into the first and second divided light and a quarter-wave plate is provided in an optical path between the beam splitter and the wavelength selective member.