The present invention addresses the problem of standardizing a front shell of a booster shell of a negative pressure-type booster device by modifying the shape of a reinforcement rib provided to the front shell so as to satisfy the demand for varying the installation pitch and installation angle of tie-rod holes with one and the same front shell. A front shell configuring a booster shell of a negative pressure-type booster device in combination with a rear shell includes a reinforcement rib formed by bending the shells, wherein the reinforcement rib is formed along a virtual ellipse of which the center is located at the center of the front shell, with the major axes of the ellipse disposed on a line connecting the centers of two tie-rod holes provided in the front shell.