Abstract: The invention relates to a structure of a liquefied hydrogen pump, located within a liquefied hydrogen fuel tank of a hydrogen (gas ignited) engine, in which a cylinder holder is linked to a lower end of the connecting rod which is reciprocated upwardly and downwardly by means of an external driving source via a first universal joint and a cylinder is housed within a piston holding member via each arm of the cylinder holder so as to be enabled to move upwardly and downwardly. A stepped engagement recess is formed in a bottom end portion of a pump housing. A bottom lid of the piston holding member is linked to an engagement rod via a second universal joint, the engagement rod being received by the engagement recess so that a radial displacement of the piston holding member and piston member to the pump housing is permitted and an automatic axial alignment of both piston and cylinder to an axial line 01--01 of the pump housing is carried out.