To provide a laser diode element having an excellent light outputting characteristic and an excellent modulating characteristic over a wide temperature range by providing an active layer composed of an InGaAsP multi-strain quantum well and specifying the resonator length, detuning amount at a room temperature, and number of quantum wells or the light confining coefficient into a quantum well layer.

After an active layer composed of an InGaAsP strain quantum well layer 12 and a barrier layer 13 is formed on an n-type InP substrate 11, a DC-PBH current constricting structure 14 and a λ/4-shifting diffraction grating 15 are formed on the active layer. In addition, an n-side electrode 16 is formed on the rear surface and a nonreflective coating 17 and a p-side pad electrode 18 are formed on the front surface. The resonator length, detuning amount at a room temperature, and number of quantum layers or the light confining efficient into a quantum well layer are respectively set at 300-600μm, -15nm to +15nm, and 8-15 layers or 3-10%. The preferable resonator length, detuning amount, number of quantum wells or the light confining coefficient are 400-500μm, -10 to +10nm, and 10-12 layers or 5-8%, respectively.