This invention aims at improving the tear strength of a film and a sheet molded from a resin composition containing microorganism-produced aliphatic polyester. This invention relates to a biodegradable polyester resin composition containing polybutylene adipate terephthalate (PBAT) in the proportion of 60 to 400 parts by weight and modified-glycerin in the proportion of 10 to 50 parts by weight based on 100 parts by weight of aliphatic polyester (P3HA) having a repeating unit represented by Formula (1): [—CHR—CH2—CO—O—] (wherein R is an alkyl group represented by CnH2n+1 and n is an integer of 1 or more and 15 or lower) and a film or a sheet obtained by molding the biodegradable polyester resin composition in which the maximum major axis of phases containing the aliphatic polyester (P3HA) measured by a transmission electron microscopy analysis-image analysis method (TEM method) is 18 μm or lower and the average value is 8 μm or lower.