A tank holding mechanism holds a gas tank that has a cylindrical portion and hemispherical-shaped domical portions formed on respective sides of the cylindrical portion. The tank holding mechanism includes: a tank accommodating portion that contacts a peripheral wall of the cylindrical portion of the gas tank; tank end-side members provided near respective ends of the gas tank; a tank band that pushes the gas tank against the tank accommodating portion; and band securing portions that secure end portions of the tank band to the respective tank end-side members. The tank band is routed from one of the domical portions through the cylindrical portion to the other domical portion. The tank band extends along a diagonal line of the gas tank over an outer face of the cylindrical portion, conforms to the outer face of the cylindrical portion, and conforms to hemispherical outer faces of both of the domical portions.