A method of manufacturing a buffer layer of a photoelectric conversion device having a stacked structure in which a lower electrode, a photoelectric conversion semiconductor layer that generates a current by absorbing light, the buffer layer, a translucent conductive layer, and an upper electrode are stacked on a substrate, the method comprising: a preparation step for preparing a reaction solution which includes a component of at least one kind of zinc source, a component of at least one kind of sulfur source, a component of at least one kind of citrate compound, a component of at least one kind selected from the group consisting of ammonia and ammonium salt, and water, wherein: the concentration of the component is 0.001 to 0.25M; and the pH before the start of reaction is 9.0 to 12.0; and a film forming step for forming, using the reaction solution, a Zn compound layer by a liquid phase method with a reaction temperature of 70 to 95°C, wherein, in the preparation step, an aqueous solution which includes the component, an aqueous solution which includes the component, an aqueous solution which includes the component, and an aqueous solution which includes the component are prepared separately, then the aqueous solutions,, and are mixed to obtain a mixed solution, and the aqueous solution is mixed in the mixed solution, characterized in that the method further comprises a fine particle layer forming step for forming a fine particle layer of a plurality of one kind or two or more kinds of fine particles consisting primarily of at least one kind selected from the group consisting of ZnS, Zn(S, O), and Zn(S, O, OH) on the photoelectric conversion semiconductor layer; and in that the Zn compound layer consists primarily of Zn(S, O) and/or Zn(S, O, OH), and is formed on the fine particle layer, and the concentration of the component in the reaction solution is 0.41 to 1.0M. a preparation step for preparing a reaction solution which includes a component of at least one kind of zinc source, a component of at least one kind of sulfur source, a component of at least one kind of citrate compound, a component of at least one kind selected from the group consisting of ammonia and ammonium salt, and water, wherein: the concentration of the component is 0.001 to 0.25M; and the pH before the start of reaction is 9.0 to 12.0; and a film forming step for forming, using the reaction solution, a Zn compound layer by a liquid phase method with a reaction temperature of 70 to 95°C, wherein, in the preparation step, an aqueous solution which includes the component, an aqueous solution which includes the component, an aqueous solution which includes the component, and an aqueous solution which includes the component are prepared separately, then the aqueous solutions,, and are mixed to obtain a mixed solution, and the aqueous solution is mixed in the mixed solution, characterized in that the method further comprises a fine particle layer forming step for forming a fine particle layer of a plurality of one kind or two or more kinds of fine particles consisting primarily of at least one kind selected from the group consisting of ZnS, Zn(S, O), and Zn(S, O, OH) on the photoelectric conversion semiconductor layer; and in that the Zn compound layer consists primarily of Zn(S, O) and/or Zn(S, O, OH), and is formed on the fine particle layer, and the concentration of the component in the reaction solution is 0.41 to 1.0M. the concentration of the component is 0.001 to 0.25M; and the pH before the start of reaction is 9.0 to 12.0; and a film forming step for forming, using the reaction solution, a Zn compound layer by a liquid phase method with a reaction temperature of 70 to 95°C, wherein, in the preparation step, an aqueous solution which includes the component, an aqueous solution which includes the component, an aqueous solution which includes the component, and an aqueous solution which includes the component are prepared separately, then the aqueous solutions,, and are mixed to obtain a mixed solution, and the aqueous solution is mixed in the mixed solution, characterized in that the method further comprises a fine particle layer forming step for forming a fine particle layer of a plurality of one kind or two or more kinds of fine particles consisting primarily of at least one kind selected from the group consisting of ZnS, Zn(S, O), and Zn(S, O, OH) on the photoelectric conversion semiconductor layer; and in that the Zn compound layer consists primarily of Zn(S, O) and/or Zn(S, O, OH), and is formed on the fine particle layer, and which includes the component, an aqueous solution which includes the component, an aqueous solution which includes the component, and an aqueous solution which includes the component are prepared separately, then the aqueous solutions,, and are mixed to obtain a mixed solution, and the aqueous solution is mixed in the mixed solution, characterized in that the method further comprises a fine particle layer forming step for forming a fine particle layer of a plurality of one kind or two or more kinds of fine particles consisting primarily of at least one kind selected from the group consisting of ZnS, Zn(S, O), and Zn(S, O, OH) on the photoelectric conversion semiconductor layer; and in that the Zn compound layer consists primarily of Zn(S, O) and/or Zn(S, O, OH), and is formed on the fine particle layer, and the concentration of the component in the reaction solution is 0.41 to 1.0M.