A method of manufacturing a tandem thin-film solar cell is provided, the solar cell including a plurality of photoelectric conversion units stacked on a substrate, the photoelectric conversion units each having a p-type layer, an i-type photoelectric conversion layer and an n-type layer deposited in this order from a light-incident side of the solar cell, and at least a rear unit among the photoelectric conversion units that is furthest from the light-incident side being a crystalline unit including a crystalline i-type photoelectric conversion layer. The manufacturing method includes the steps of forming at least one of the units on the substrate by plasma CVD and immediately thereafter forming an i-type boundary layer to a thickness of at most 5 nm by plasma CVD, and thereafter removing the substrate into the atmosphere to expose a surface of the i-type boundary layer to the atmosphere and then forming a crystalline unit on the i-type boundary layer by plasma CVD.