To provide stannous oxide particles having a stannic oxide content of a reduced level even after long-term storage, which is usable for adjusting a tin plating bath, and which does not impart an organic material undesired for a tin (II) ion-containing plating bath.

This method is characterized in that (a) a plurality of stannous oxide particles are provided, and at least a part of the plurality of particles includes a surface coating of stannic oxide, and (b) the plurality of particles are treated with a reducing agent over a period sufficient to reduce at least a part of stannic oxide coatings into stannous oxide without forming substantially metal tin.