A semiconductor stack includes a semiconductor DBR (Distributed Bragg Reflector) formed on a substrate, and a resonator formed on the semiconductor DBR laminating wide-band semiconductor layers and active layers alternately. Each of the active layers includes MQWs (Multiple Quantum Wells) and two spacer layers formed one on each surface of the MQWs. The MQWs are formed by laminating barrier layers and quantum well layers alternately. There are n layers of the wide-band semiconductor layer formed, and a band gap Egm of an m-th wide-band semiconductor layer counting from the substrate and a band gap Egm-1 of an m−1-th wide-band semiconductor layer counting from the substrate satisfy Egm-1