A fluctuation in color or the like is prevented when medical images are displayed in time series. For that purpose, a registration unit (30) correlates voxel positions in sets of three-dimensional volume data (100) constituting a three-dimensional volume data group (110) by performing registration between the sets of three-dimensional volume data (100) . A conversion unit (40) selects base phase B, and converts a signal value at each voxel position in all the sets of three-dimensional volume data (100) into a signal value at a corresponding voxel position in a base three-dimensional volume data (120) in the base phase B. Further, the conversion unit (40) converts the signal value in each of the sets of three-dimensional volume data (100) into a display signal value, using color template T0.
