The present invention provides a method of transporting parts constituting a product in a product assembly plant, the method including placing the parts in a returnable container, the returnable container being carried by a worker within the plant, wherein the returnable container is formed by expansion molding of expanded particles of a polyolefin-based resin, the relationship between the weight and volume of the returnable container satisfies Formula (1) below, and the relationship between the flexural modulus and density of the returnable container satisfies Formula (2) below: 650≦(a−W)/W×V≦4,000  (1) (where W is the weight (kg) of the returnable container, V is the volume (L) of the returnable container, and a represents 23 kg, i.e., the maximum weight that can be carried by a worker within the plant, which is recommended by the National Institute for Occupational Safety & Health (NIOSH)); 0.10≦F/D≦0.60  (2) (where D is the density (g/L) of the returnable container, and F is the flexural modulus (MPa) measured according to ISO 1209).