A semiconductor acceleration sensor including a trench provided in a main surface of a semiconductor substrate, the trench having a first inner wall, a second inner wall opposite to the first inner wall, and a third inner wall joining the first and second inner walls. A gate electrode faces the first, second and third inner walls of the trench through an air gap. A first semiconductor unit is formed in the first inner wall consisting of three adjoining semiconductor layers for detecting a displacement of the gate electrode relative to the first semiconductor unit induced by an applied acceleration, each of the three adjoining semiconductor layers in the first semiconductor unit having a different conductivity type. A second semiconductor unit is formed in the second inner wall consisting of three adjoining semiconductor layers for detecting a displacement of the gate electrode relative to the second semiconductor unit induced by an applied acceleration, each of the three adjoining semiconductor layers in the second semiconductor unit having a different conductivity type.