Disclosed herein is a semiconductor laser device utilizing a sub-mount substrate that is capable of having a further sufficient heat dissipation property. The semiconductor laser device comprises: a monocrystalline sub-mount substrate having a crystalline structure including a first crystalline plane (c-plane) having a normal line direction on a first crystalline axis (c-axis) and a second crystalline plane (a-plane) having a normal line direction on a second crystalline axis (a-axis) having a higher thermal conductivity than the first crystalline axis; and a semiconductor laser chip configured to be joined to a side of a first surface of the sub-mount substrate. The first crystalline plane inclines with respect to the first surface of the sub-mount substrate.