To eliminate degradation factors due to the wetting layer of a quantum well type, to improve light emission efficiency and wavelength multiplex, and to prevent a dark current.

A quantum semiconductor device has a quantum semiconductor structure provided with (a) a thin film crystal layer positioned on a base semiconductor layer and constituted of a material capable of growth in an S-K mode to the base semiconductor layer without constituting a quantum well to the base semiconductor layer and (b) a quantum dot positioned right above the thin film crystal layer and constituting type I energy band discontinuity to the base semiconductor layer. The thin film crystal layer is constituted of a material to be type II energy band discontinuity to the base semiconductor layer or a material having a band gap larger than the base semiconductor layer.

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