An exhaust valve device for a fuel tank; comprising: a casing provided to pass through an upper wall of a fuel tank, and partitioning an inside into an upper chamber and a lower chamber by a bulkhead including a through hole; an inflow hole communicating the lower chamber and an inside of the fuel tank; an exhaust hole communicating a side portion of the upper chamber and an outside of the fuel tank; and a float valve displaceably supported in the lower chamber, and ascending by receiving buoyancy from a fuel flown into the lower chamber through the inflow hole so as to close the through hole, wherein a portion defining the upper chamber of the casing includes a cylinder portion and a circular plate closing an upper end of the cylinder portion, the exhaust hole is formed in the cylinder portion, on an upper chamber side of the bulkhead, a cylinder body whose upper portion is closed to cover the through hole is provided approximately in a same shaft as the cylinder portion, and a buffer chamber is defined between the cylinder body and the bulkhead, the buffer chamber communicates with the lower chamber by the through hole, in a side wall on a side opposing an exhaust hole side of the cylinder body, a first ventilation hole communicating the buffer chamber and the upper chamber is formed, characterized in that at a boundary portion between an inner circumferential face of the cylinder portion and a lower face of the circular plate, a corner portion having a curved surface shape smoothly connecting between the inner circumferential face of the cylinder portion and the lower face of the circular plate is extended, and a curvature of the corner portion is smallest on a side opposing a side forming the exhaust hole, and gradually increases as moving toward a side forming the exhaust hole in a circumferential direction. a casing provided to pass through an upper wall of a fuel tank, and partitioning an inside into an upper chamber and a lower chamber by a bulkhead including a through hole; an inflow hole communicating the lower chamber and an inside of the fuel tank; an exhaust hole communicating a side portion of the upper chamber and an outside of the fuel tank; and a float valve displaceably supported in the lower chamber, and ascending by receiving buoyancy from a fuel flown into the lower chamber through the inflow hole so as to close the through hole, wherein a portion defining the upper chamber of the casing includes a cylinder portion and a circular plate closing an upper end of the cylinder portion, the exhaust hole is formed in the cylinder portion, on an upper chamber side of the bulkhead, a cylinder body whose upper portion is closed to cover the through hole is provided approximately in a same shaft as the cylinder portion, and a buffer chamber is defined between the cylinder body and the bulkhead, the buffer chamber communicates with the lower chamber by the through hole, in a side wall on a side opposing an exhaust hole side of the cylinder body, a first ventilation hole communicating the buffer chamber and the upper chamber is formed, characterized in that at a boundary portion between an inner circumferential face of the cylinder portion and a lower face of the circular plate, a corner portion having a curved surface shape smoothly connecting between the inner circumferential face of the cylinder portion and the lower face of the circular plate is extended, and a curvature of the corner portion is smallest on a side opposing a side forming the exhaust hole, and gradually increases as moving toward a side forming the exhaust hole in a circumferential direction.