An object and a problem of the present invention is to provide tin(II) oxide powder which has extremely high solubility in an acid or an acidic plating solution, excellent in storage stability in the air and can heighten oxidation-preventive effect of Sn 2+ ion in the plating solution. The method for manufacturing tin(II) oxide powder of the present invention comprises Step (11) of preparing an aqueous acidic solution containing Sn 2+ ions, Step (12) of neutralizing the aqueous acidic solution by adding an aqueous alkaline solution to prepare a slurry of tin(II) hydroxide, Step (13) of dehydrating the prepared slurry to obtain a slurry of tin(II) oxide, Step (14) of separating the slurry of tin(II) oxide into a solid and a liquid to obtain tin(II) oxide, Step (15) of treating the obtained tin(II) oxide with an aqueous antioxidant solution, and Step (16) of vacuum drying the tin(II) oxide treated with the aqueous antioxidant solution. Also, the tin(II) oxide powder of the present invention is suitable for replenishing a tin component to a tin-alloy plating solution.