this embodiment however the interpolation is carried out on the basis of case a shown in the above table that is the interpolation data is obtained as fr 2 a1 b1 2 thus if a1 b1 the interpolation data is fr 2 so that it is displayed as blue color where the sample data a1 and b1 respectively represent blue color and red color a1 b1 the interpolation data is not zero so that no black frame is provided fig8 shows this case it will be seen that the interpolation data never becomes zero when the r and u03b8 direction interpolation data obtained in the above way are stored together with the sampled data in the frame memory they are stored in corresponding memory elements as shown shaded in fig9 in this case no data is present memory elements other than those in which the sampled data and r and u03b8 direction interpolation data are stored thus when image data is read out from the frame memory interpolation i e h direction interpolation is performed in the horizontal scanning direction of the monitor and interpolation data h1 to h40 are stored in vacant memory elements if the data for interpolation are b mode image data and data representing the variance value u03c3 these data have no signs so that the interpolation is performed on the basis of case d now the timing of interpolation will be described with reference to fig1 to 12 fig1 shows the timing of interpolation in the r direction in the period pulse period of a start timing clock rate of.