The present invention aims to provide a primer composition that can give good adhesion between an acrylic resin and an inorganic substrate, such as a substrate formed of metal, metal oxide, glass, ceramic, cement, or mortar and is suitably used as a primer for bonding them together. The primer composition is produced by mixing (a) a polythiol having at least two mercapto groups in one molecule, (b) a silane compound having at least one functional group and at least one alkoxysilyl group in one molecule, the at least one functional group being reactive with a mercapto group, and water.