It possible to provide an ultrasonogram having a preferable spatial resolution and signal/noise ratio. A signal dynamic range is measured so that selection between a linear interpolation or sinc function interpolation is adaptively performed for the signal or weighted averaging of the both is performed. It is possible to set in advance a depth position for switching between the linear interpolation and the sinc function interpolation in a storage unit (21) so that the two interpolation methods are switched from one to the other at the boundary of the depth position.