To simplify a circuit configuration without requiring orthogonality of input shafts.

In this sensor, capacities Cx1, Cx2 differentially vary by an x-axis acceleration, a clock CLK charges and discharges the capacities Cx1, Cx2 through resistors 35x, 36x, a circuit 38x executes exclusive OR calculation of voltage waveforms of the capacities Cx1, Cx2 to output a square wave in the case of the presence of the x-axis acceleration and to output a low level in the case of the absence thereof. With respect to y-axis acceleration and z-axis acceleration, similar outputs are respectively obtained from circuits 38y, 38z. The respective outputs from the circuits 38x, 38y, 38z are supplied to an NOR circuit 41, and the NOR circuit 41 outputs a gravity-free state only in the case of the absence of all the three-axis accelerations.