ds are performed sequentially since the durations of respective drive signals ds are of the magnitude of microseconds processing pertaining to the foregoing processes can be performed within a considerably short time period the natural period calculator 44 of the natural period measuring apparatus 40 again prepares a plotted graph on the basis of the plurality of types of corrected drive signals ds and the velocity of a liquid droplet obtained by the velocity measuring apparatus 10 wherein the duration of the first voltage holding section s2 is taken as a horizontal axis and the velocity of a liquid droplet is taken as a vertical axis from such a plotted graph the natural period calculator 44 of the natural period measuring apparatus 40 specifies two peak values in the duration of the first voltage holding section s2 with regard to the velocity of a liquid droplet a difference between the durations of the two first voltage holding sections corresponding to the two peak values is output as a natural period tc of the liquid stored in the pressure generating chamber 52 the comparator 45 again compares the natural period tc obtained by the natural period calculator 44 with the predicted design natural.