The present application provides: a transformed microorganism for producing a PHA copolymer containing 3HH monomer unit at a higher composition ratio, specifically, a transformed microorganism comprising a PHA synthase gene capable of synthesizing a PHA copolymer containing 3HH monomer unit and a gene encoding a protein having (R)-specific enoyl-CoA hydratase activity, characterized in that, in the transformed microorganism, the expression of a gene encoding at least one β-ketothiolase enzyme having thiolysis activity for β-keto-(C6) acyl-CoA (i.e., β-ketohexanoyl-CoA) is inhibited, thereby losing or reducing the enzyme activity; and a method for producing a PHA copolymer containing 3HH monomer unit, comprising a step of culturing the transformed microorganism.