A high-pressure vessel unit includes: plural cylindrical vessels arrayed inside a case, with end portions of the vessels on one side in the axial direction thereof being equipped with openings; a coupling member connected to the openings of the vessels to couple the plural vessels and includes a flow passage that communicates the insides of the vessels; a lead-out pipe that is leads out to the outside of the case from the coupling member through a through hole formed in the case; securing members that secure the coupling member to the case; and a retention mechanism that retains portions of the vessels on the axial direction other side of the end portions on the one side in the axial direction such that those portions of the vessels on the axial direction other side are movable in the axial direction relative to the case.