A resin fuel tank includes a fuel tank main body (19) in which a closed space for storing fuel is formed in its interior; a built-in part (24) that is disposed within the closed space and is fixed to a wall portion of the fuel tank main body by at least two mounting portions (46); connecting portions (32, 34) of the built-in part, which are separated from the wall portion and that connect the two mounting portions (46); and deformation absorbing portions (48A, 48B) of the built-in part which are elastically deformable, are upside-down U-shaped as seen in side view and are provided between the mounting portions (46) and the connecting portions (32, 34), one ends (68A, 68B) of the deformation absorbing portions being fixed to the connecting portions, and other ends (56A, 56B) of the deformation absorbing portions being separated from the connecting portions in a height direction.