PURPOSE: To obtain an image contour stress processing which allows the setting of a contour for a contour stress processing freely by enabling the setting of an optional area desired for the contour stress processing on an image and a frame surrounding the area.

CONSTITUTION: When σ dispersion value σ is compared with two reference dispersion values σ1 and σ2, a specified ultrasonic signal is transmitted to or received with an object 24 to be inspected from a probe 10 and an echo signal received is stored by one screen portion serial into a memory 12 as image information. Image density level information stored is outputted as a notice pixel value (z) and a means 2 in an area near pixels of ×n containing (z). But the dispersion value σ is determined based on the means near the pixels of n×n in a specified area and a square mean. A comparison/computation circuit 18 compares the dispersion value σ with two different reference values σ1 and σ2 predetermined and a pixel value stressing circuit 22 performs a contour stress processing to set the notice pixel value larger when the dispersion value is larger than the pixel mean and smaller when it is smaller than the mean. This enables the setting of a specified area alone thereby obtaining a pixel display state subjected to a proper contour stress processing.