The present invention relates to powder containing silver nanoparticles having an average particle size of 2 to 50 nm in an amount of 95% or more by mass and to applications of the powder. Silver-containing powder containing the silver nanoparticles is obtained by reducing a silver compound in the presence of a compound obtained by bonding polyethylene glycol to polyethyleneimine having a certain molecular weight and then by performing a concentration step and a drying step. A plastic substrate is obtained by directly applying a conductive paste that uses the powder on a plastic substrate and by performing drying.