Disclosed is a method for manufacturing a thin-film solar cell using plasma between a couple of parallel electrodes. In the method, a base member is placed in a chamber between a first electrode and a second electrode facing each other. A hydrogen gas is heated, and thus heated hydrogen gas and a silicon-based gas are introduced into a space between the first electrode and the second electrode. A ratio of a flow rate of the heated hydrogen gas to that of the silicon-based gas is at least 25 and no more than 58. A plasma is generated between the first electrode and the second electrode by applying high-frequency power to the second electrode while a pressure in the chamber is 1000 Pa or higher, and an optically active layer containing crystalline silicon is deposited on the base material.