A biaxially stretch blow molded container is formed with a clearly transparent window portion in a longitudinal strip shape by preventing mixture and cut-in of a colored resin to the window portion. In a predetermined range extending from an upstream end to a downstream position of a flow path including a cylindrical flow path and a reduced-diameter flow path formed in a nozzle portion, a pair of guiding ribs in the form of longitudinal ridges is linearly arranged partitioning the flow path in a circumferential direction, and a longitudinal groove flow path is formed between the guiding ribs. A molten B resin is supplied to the longitudinal groove flow path, and a molten A resin is supplied to the cylindrical flow path excluding the longitudinal groove flow path. The longitudinal strip-shaped flow path of the B resin interrupts the cylindrical flow path of the A resin in the circumferential direction.