A thickness measuring device using ultrasonic waves is provided. A cortical bone thickness measuring device includes a plurality of oscillators, a reception waveform storage, an echo waveform synthesizing module, an inner-surface focusing waveform acquiring module, and a thickness calculating module. The plurality of oscillators are arrayed and each of the oscillators is transmittable and receivable of an ultrasonic wave. The echo waveform synthesizing module obtains an echo waveform corresponding to the ultrasonic beams by synthesizing reception waveforms of the respective oscillators stored in the reception waveform storage in advance, while scanning a focusing position of the ultrasonic beams. Concerning the obtained echo waveform, when it is determined that the beams suitably focus on an inner surface of a cortical bone as a result of an evaluation by the inner-surface focusing waveform acquiring module, the thickness calculating module calculates the thickness of the cortical bone based on the echo waveform.
