A tank includes a tank wall that has an opening, which is configured into a generally rectangular form that extends in a longitudinal direction and a lateral direction while a cross section of the tank wall, which is perpendicular to the longitudinal direction of the opening, is curved, and a tank foot is formed to extend all around the opening of the tank wall. A gate position is placed in a longitudinal center portion of one side surface of the tank foot, which extends in the longitudinal direction of the tank wall.