A semiconductor laser device comprising: a first substrate having a semiconductor laser formed thereon which oscillates in a single mode to emit output light; wherein the first substrate includes: branching means for branching the output light from the semiconductor laser into two, and generating a first part of the output light from the semiconductor laser as a first branched light and generating a second part of the output light of the semiconductor laser device as a second branched light; and a first semiconductor optical amplifier configured to amplify the first branched light of the branching means such that the amplified first branched light is output on a first output end side of the first substrate, and a second semiconductor optical amplifier configured to amplify the second branched light of the branching means, a second substrate being composed of Si and having a lightwave circuit formed thereon which is configured to propagate the amplified second branched light through a fixed length of an optical path and then to feed the amplified second branched light back via the second amplifier to the semiconductor laser; and a third substrate having the first substrate and the second substrate mounted thereon, wherein a second output end side of the first substrate on which the amplified second branched light is emitted and an input waveguide of the lightwave circuit on the second substrate are optically connected to each other. a first substrate having a semiconductor laser formed thereon which oscillates in a single mode to emit output light; wherein the first substrate includes: branching means for branching the output light from the semiconductor laser into two, and generating a first part of the output light from the semiconductor laser as a first branched light and generating a second part of the output light of the semiconductor laser device as a second branched light; and a first semiconductor optical amplifier configured to amplify the first branched light of the branching means such that the amplified first branched light is output on a first output end side of the first substrate, and a second semiconductor optical amplifier configured to amplify the second branched light of the branching means, a second substrate being composed of Si and having a lightwave circuit formed thereon which is configured to propagate the amplified second branched light through a fixed length of an optical path and then to feed the amplified second branched light back via the second amplifier to the semiconductor laser; and a third substrate having the first substrate and the second substrate mounted thereon, wherein a second output end side of the first substrate on which the amplified second branched light is emitted and an input waveguide of the lightwave circuit on the second substrate are optically connected to each other. branching means for branching the output light from the semiconductor laser into two, and generating a first part of the output light from the semiconductor laser as a first branched light and generating a second part of the output light of the semiconductor laser device as a second branched light; and a first semiconductor optical amplifier configured to amplify the first branched light of the branching means such that the amplified first branched light is output on a first output end side of the first substrate, and a second semiconductor optical amplifier configured to amplify the second branched light of the branching means, a second substrate being composed of Si and having a lightwave circuit formed thereon which is configured to propagate the amplified second branched light through a fixed length of an optical path and then to feed the amplified second branched light back via the second amplifier to the semiconductor laser; and a third substrate having the first substrate and the second substrate mounted thereon, wherein a second output end side of the first substrate on which the amplified second branched light is emitted and an input waveguide of the lightwave circuit on the second substrate are optically connected to each other.