1. A medical image processing apparatus comprising: at least one data processor coupled with digital storage, an operator input port and an operator display output port, said processor being configured to acquire plural series of image data, each series including frames of medical image data corresponding to at least one of slice positions and time phases; and display image data in a region of interest of at least one frame of first medical image data belonging to a first series out of the plural series, the image data in said region of interest being overlapped with a different region of at least one frame of second medical image data belonging to a second series out of the plural series, the different region of the at least one frame of the second medical image data being different from a region corresponding to the region of interest wherein the second medical image data, in which the image data in the region of interest in the first medical image data are overlapped with the region different from the region corresponding to the region of interest are switched and displayed in a slice direction or a time phase direction in the second series at a switching frequency different from a switching frequency of the image data in the region of the interest, the second medical image data being switched and displayed responsive to an instruction for switching the first medical image data in a slice direction or a time phase direction in the first series, the instruction being input from the operator input port.