A blow mold unit comprising: first and second blow cavity split molds (12, 14) to be clamped together by bringing their parting faces (12A, 14A) into abutment with each other; a plurality of bottom molds defining a plurality of bottom shapes for a plurality of cavities which are defined by the first and second blow cavity split molds (12, 14); first pressure receiving plates respectively fixed to both side faces of the first blow cavity split mold, which are located at both ends in a first direction along a direction in which the plurality of cavities are aligned; second pressure receiving plates respectively fixed to both side faces of the second blow cavity split mold and brought into abutment with the first pressure receiving plates when the first and second blow cavity split molds (12, 14) are clamped together; a first fixing plate to which the first blow cavity split mold and the first pressure receiving plates are fixed; a second fixing plate to which the second blow cavity split mold and the second pressure receiving plates are fixed; a bottom mold fixing plate to which the plurality of bottom molds are fixed; the blow molding unit being characterized in pressure receiving members respectively fixed to the first and second fixing plates (40, 50) and brought into engagement with the bottom mold fixing plate when the first and second blow cavity split molds (12, 14) are clamped together to receive a blowing pressure exerted on the bottom molds; guide plates respectively fixed to the bottom mold fixing plate at both end portions in the first direction; and guided members respectively supported by the first and second pressure receiving plates (20, 30) at both end portions in the first direction and capable of being brought into abutment with the guide plates when the bottom molds are in a closed position, wherein a length of the guide plates in a second direction, which is a direction in which the first and second blow cavity split molds (12,14) are opened and closed, is configured so that the guided members face the guide plates at least immediately before the pressure receiving members are brought into engagement with the bottom mold fixing plate when the first and second blow cavity split molds (12, 14) are clamped together. first and second blow cavity split molds (12, 14) to be clamped together by bringing their parting faces (12A, 14A) into abutment with each other; a plurality of bottom molds defining a plurality of bottom shapes for a plurality of cavities which are defined by the first and second blow cavity split molds (12, 14); first pressure receiving plates respectively fixed to both side faces of the first blow cavity split mold, which are located at both ends in a first direction along a direction in which the plurality of cavities are aligned; second pressure receiving plates respectively fixed to both side faces of the second blow cavity split mold and brought into abutment with the first pressure receiving plates when the first and second blow cavity split molds (12, 14) are clamped together; a first fixing plate to which the first blow cavity split mold and the first pressure receiving plates are fixed; a second fixing plate to which the second blow cavity split mold and the second pressure receiving plates are fixed; a bottom mold fixing plate to which the plurality of bottom molds are fixed; the blow molding unit being characterized in pressure receiving members respectively fixed to the first and second fixing plates (40, 50) and brought into engagement with the bottom mold fixing plate when the first and second blow cavity split molds (12, 14) are clamped together to receive a blowing pressure exerted on the bottom molds; guide plates respectively fixed to the bottom mold fixing plate at both end portions in the first direction; and guided members respectively supported by the first and second pressure receiving plates (20, 30) at both end portions in the first direction and capable of being brought into abutment with the guide plates when the bottom molds are in a closed position, wherein a length of the guide plates in a second direction, which is a direction in which the first and second blow cavity split molds (12,14) are opened and closed, is configured so that the guided members face the guide plates at least immediately before the pressure receiving members are brought into engagement with the bottom mold fixing plate when the first and second blow cavity split molds (12, 14) are clamped together.