An LD module, comprising: a multi-transverse-mode laser diode , including an active player , which emits a laser beam from an exit end surface ; a multi-mode fiber , including a core , whose entrance end surface the laser beam enters; and an optical system provided between the multi-mode laser diode and the multi-mode fiber, wherein the optical system includes: a collimating lens for refracting, within a plane parallel to the active layer, the laser beam which is divergent light emitted from the multi-mode laser diode so as to convert the laser beam into parallel light; and a light-converging lens for converting, into convergent light, the parallel light which has exited from the collimating lens, the multi-mode laser diode has an emitter width greater than a diameter, of the core, in the entrance end surface of the multi-mode fiber, the optical system converges the laser beam so that a diameter, of the laser beam, in the entrance end surface of the multi-mode fiber becomes smaller than the diameter, of the core, in the entrance end surface of the multi-mode fiber, the multi-mode laser diode is configured so that a beam parameter product of the laser beam emitted from the multi-mode laser diode when optical output power is constant shows a local minimum value as a function of the emitter width of the multi-mode laser diode, the local minimum value being equal to or smaller than a beam parameter product of the multi-mode fiber, and the emitter width of the multi-mode laser diode is set so that the beam parameter product of the laser beam emitted from the multi-mode laser diode is equal to the local minimum value. a multi-transverse-mode laser diode , including an active player , which emits a laser beam from an exit end surface ; a multi-mode fiber , including a core , whose entrance end surface the laser beam enters; and an optical system provided between the multi-mode laser diode and the multi-mode fiber, wherein the optical system includes: a collimating lens for refracting, within a plane parallel to the active layer, the laser beam which is divergent light emitted from the multi-mode laser diode so as to convert the laser beam into parallel light; and a light-converging lens for converting, into convergent light, the parallel light which has exited from the collimating lens, a collimating lens for refracting, within a plane parallel to the active layer, the laser beam which is divergent light emitted from the multi-mode laser diode so as to convert the laser beam into parallel light; and a light-converging lens for converting, into convergent light, the parallel light which has exited from the collimating lens, the multi-mode laser diode has an emitter width greater than a diameter, of the core, in the entrance end surface of the multi-mode fiber, the optical system converges the laser beam so that a diameter, of the laser beam, in the entrance end surface of the multi-mode fiber becomes smaller than the diameter, of the core, in the entrance end surface of the multi-mode fiber, the multi-mode laser diode is configured so that a beam parameter product of the laser beam emitted from the multi-mode laser diode when optical output power is constant shows a local minimum value as a function of the emitter width of the multi-mode laser diode, the local minimum value being equal to or smaller than a beam parameter product of the multi-mode fiber, and the emitter width of the multi-mode laser diode is set so that the beam parameter product of the laser beam emitted from the multi-mode laser diode is equal to the local minimum value. <EOS>