Disclosed herein is a long wavelength multiple quantum well semiconductor laser which is capable of preventing the nonuniform injection of carriers into quantum wells, and the overflow of the electrons within the quantum wells, and which is also capable of suppressing the increase in the threshold caused by insufficient gain, and the increase in the threshold and the deterioration in the slope efficiency caused by internal absorption loss. In a multiple quantum well semiconductor laser which has an InGaAsP optical waveguide layer provided respectively on the inside of a p-InP clad layer and an n-InP clad layer, and InGaAsP barrier layers and strained InGaAsP quantum well layers provided between the InP clad layers, the semiconductor laser according to this invention has the energy difference between the first quantum level of the hole in the quantum well and the top of the valence band in the barrier layer to be less than or equal to 160 meV, the energy difference between the first quantum level of the electron in the quantum well and the bottom of the conduction band in the barrier layer to be more than or equal to 30 meV, and the optical confinement factor to the quantum layer lying in the range from 0.01 to 0.07. IMAGE
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