The present invention is to provide a semiconductor acceleration sensor capable of sensing accelerations in two directions parallel to the surface of a diaphragm and orthogonal to each other with respective proper sensitivities. A semiconductor acceleration sensor is constituted of diaphragm pieces extending from the center of the diaphragm surface to a wafer outer-circumferential frame section, respectively, along an X axis direction and a Y axis direction orthogonal to each other. On the upper surface of the diaphragm pieces, there are formed piezo resistors Rx1 to Rx4, Ry1 to Ry4, Rz1 to Rz4. In the diaphragm pieces disposed on a single line along the X axis direction and the diaphragm pieces disposed on a single line along the Y axis direction, the areas of cross section orthogonal to the axis are set according to a maximum value of acceleration, respectively, in the X axis direction or Y axis direction.