To provide a position measurement device capable of measuring a target position with high accuracy according to an ultrasonic image during treatment, a treatment system including the device, and a position measurement method. A first image is constructed according to an ultrasonic waveform acquired by an ultrasonic sensor 104C, a three-dimensional image acquired in advance and the first image are collated on the basis of sensor position information acquired by a sensor position measurement unit 105 to calculate sound velocities of each body tissue of a patient 100, a second image is constructed according to the ultrasonic waveform acquired by the ultrasonic sensor 104C using the calculated sound velocities of each body tissue of the patient 100, and a target tissue position is calculated according to the second image.
