the object of the invention is to provides a process in which a high - molecular - weight β - butyrolactone homopolymer or copolymer being biodegradable , absorbable into the living body , useful as a functional polymeric material and superior in physical characteristics is produced with high yield and good productivity . this object is solved by a process for producing the homopolymer or copolymer of the invention , which comprises ring - opening polymerization of sole β - butyrolactone subjected previously to acid removal and dehydration treatment , or of said β - butyrolactone with other lactones , in the presence of a tin compound represented by the following formula and / or formula : wherein r 1 and r 2 and r 4 to r 7 independently represent an alkyl group containing 1 to 12 carbon atoms which may have a branched chain , or a cycloalkyl group containing 5 to 7 carbon atoms , or a phenyl or naphthyl group which may have a substituent group , r 3 represents an alkylene group containing 2 to 12 carbon atoms which may have a substituent group , a cyclopentane - 1 , 2 - ylene group or a cyclohexane - 1 , 2 - ylene group , and n is an integer of 1 to 10 .