According to an embodiment, a transmitter transmits a reference signal. A position sensor obtains position information in a three-dimensional space by receiving the reference signal. A controlling unit associates three-dimensional image data generated by a medical image diagnosis apparatus with the three-dimensional space, on the basis of a registration between an arbitrary cross-sectional plane in the three-dimensional image data and a cross-sectional plane scanned by an ultrasound probe. A detecting unit detects if the position of the transmitter has changed within the associated three-dimensional space, on the basis of the position information obtained by the position sensor. A correcting unit corrects a misregistration between a cross-sectional plane in the three-dimensional image data and a cross-sectional plane scanned by the ultrasound probe, on the basis of a change amount of the position of the transmitter.