1. A method for estimating a profile depth of a tire of a vehicle during an operation of the vehicle, which comprises the steps of: measuring signals of a piezoelectric element disposed on an inside of the tire in a region that is deformed when passing through a contact area; deriving the actual data from the signals measured by determining a first summation value by accumulating signals corresponding to a gentler curvature of the piezoelectric element than a normal curvature of the piezoelectric element or determining a second summation value by accumulating signals corresponding to a tighter curvature of the piezoelectric element than the normal curvature of the piezoelectric element; comparing the actual data derived from the signals with comparison data, the comparison data containing records that indicate expected actual data for each of a plurality of profile depths; and estimating the profile depth based on a comparison of the signals or the actual data with the comparison data.