A digital scan converter employed in a sector scanning ultrasound diagnostic device has a digital memory for storing a received image signal in a cell whose address corresponds to the picture element and a system controller for reading out a signal from the memory by linear scanning. The digital scan converter is further provided with a signal generator and an adder for shifting the level of the image signal by a predetermined value when the signal is written in the memory, a level detector for detecting whether or not the signal read out from the memory is at level "0" and an interpolating section for converting the output signal from the memory into an interpolating signal by the output from the level detector.