A fuel tank for a saddle riding vehicle, comprising: an upper tank half body that is made of metal and press-formed with an upper joint flange on an outer periphery of the upper tank half body; and a lower tank half body that is made of metal and press-formed with a lower joint flange on an outer periphery of the lower tank half body, the lower joint flange being joined to the upper joint flange, wherein parts of the lower joint flange and the upper joint flange are formed in a flat shape as a lower planar flange portion and an upper planar flange portion, respectively, and the lower tank half body is formed with: a lower tank main body portion that is elongated in a direction parallel to a reference plane including joint surfaces of the lower planar flange portion and the upper planar flange portion, includes a narrow portion narrower on one side of the lower tank main body portion than an other side thereof along a longitudinal direction of the lower tank main body portion, and bulges toward a lower side from the reference plane; and a pair of expansion tank portions that are disposed on opposite sides of the narrow portion as seen in a plan view from a direction orthogonal to the reference plane, formed to be shallower than the lower tank main body portion, and bulge downward from the reference plane, characterized in that a lower tank main body deepest portion of the lower tank main body portion and expansion tank deepest portions of the expansion tank portions are disposed at positions offset from each other in the longitudinal direction, the lower tank main body deepest portion being largest in distance from the reference plane, the expansion tank deepest portions being largest in distance from the reference plane. an upper tank half body that is made of metal and press-formed with an upper joint flange on an outer periphery of the upper tank half body; and a lower tank half body that is made of metal and press-formed with a lower joint flange on an outer periphery of the lower tank half body, the lower joint flange being joined to the upper joint flange, wherein parts of the lower joint flange and the upper joint flange are formed in a flat shape as a lower planar flange portion and an upper planar flange portion, respectively, and the lower tank half body is formed with: a lower tank main body portion that is elongated in a direction parallel to a reference plane including joint surfaces of the lower planar flange portion and the upper planar flange portion, includes a narrow portion narrower on one side of the lower tank main body portion than an other side thereof along a longitudinal direction of the lower tank main body portion, and bulges toward a lower side from the reference plane; and a pair of expansion tank portions that are disposed on opposite sides of the narrow portion as seen in a plan view from a direction orthogonal to the reference plane, formed to be shallower than the lower tank main body portion, and bulge downward from the reference plane, a lower tank main body portion that is elongated in a direction parallel to a reference plane including joint surfaces of the lower planar flange portion and the upper planar flange portion, includes a narrow portion narrower on one side of the lower tank main body portion than an other side thereof along a longitudinal direction of the lower tank main body portion, and bulges toward a lower side from the reference plane; and a pair of expansion tank portions that are disposed on opposite sides of the narrow portion as seen in a plan view from a direction orthogonal to the reference plane, formed to be shallower than the lower tank main body portion, and bulge downward from the reference plane, characterized in that a lower tank main body deepest portion of the lower tank main body portion and expansion tank deepest portions of the expansion tank portions are disposed at positions offset from each other in the longitudinal direction, the lower tank main body deepest portion being largest in distance from the reference plane, the expansion tank deepest portions being largest in distance from the reference plane.