The present provides an environmental low load resin composition with excellent strength, impact resistance, heatproof, and molding, which can decrease the amount of the CO2 exhaust when manufacturing it. A resin composition is formulated by (A) styrene-based resin, (B) aliphatic polyester, and at least one of (C) phase dissolving agent and (D) dicarboxylic acid anhydride, preferably wherein (C) phase dissolving agent is one or above of followings: (C-1) poly methylmethacrylate polymer, (C-2) 3 wt.% and more vinyl-based polymer copolymerized by unsaturated carboxylic acid glycidyl ester unit or unsaturated dicarboxylic acid anhydride unit, (C-3) graft polymer graft-polymerized by 10~80wt.% gum material polymer with 20~90wt.% methylmethacrylate unit, (C-4) a block copolymer, the copolymerization ratio((I)/(II)) 99/1-1/99 parts by weight (in which (I)+(II) is 100 parts) of polylactic acid segment represented by general formula (I) and vinyl-based polymer segment represented by general formula (II), (X is at least one selected from hydrogen, alkyl, hydroxyl, alkylester group, cyano, phenyl, amide and halogen; m and n representing number average molecular weight are each independent 1000~100000).