Disclosed herein is an inertial sensor, including: a membrane; a mass body disposed under the membrane; a sensing unit formed on the membrane and including a piezoelectric body; and a spring constant control unit formed to be spaced apart from the sensing unit and including a piezoelectric body. According to the preferred embodiment of the present invention, the DC acceleration (in particular, gravity acceleration) can be measured by using the change in the spring constant without changing the structure of the inertial sensor including the piezoelectric material of the prior art.