In order to provide a CIGS compound solar cell with a high conversion efficiency, a CIGS compound solar cell including a rear electrode layer, a CIGS light absorbing layer, a buffer layer, and a transparent electrode layer in this order over a substrate is configured such that the buffer layer comprises a mixed crystal of a Group IIa metal and zinc oxide, and characteristics of the mixed crystal as shown by X-ray diffraction satisfy the following formula (1): 0.5≤A/(A+B+C)