A valve device for a fuel tank includes a float element accommodated vertically movably in a case. The float element has a seal member at an upper portion thereof. The fuel flowing into the case through the inflow portion rises the float element to seat the seal member on the vent valve port. A ventilation portion connecting between inside of the tank and inside of the case is formed at an upper face portion of the case positioned above a waterline of the float element when seated. The upper face portion of the float element positioned in close proximity to the upper face portion reduces the amount of ventilation through the ventilation portion when seated. The valve device for a fuel tank further includes a regulation portion at the upper face portion of the case in which the interval is constant between the upper face portion of the case and the upper face portion of the float element when seated.