PURPOSE: To improve the efficiency of a data transfer without hindering the general applicability of an interface device transferring image data composed of binary data of characters, etc., and multi-valued data of patterns, etc., from a computer, etc., to a printer, etc.

CONSTITUTION: All the image data is outputted by the number of bit of multivalued data to an external device 10 even when binary data and multi- valued data are transferred by each proper number of bit to improve efficiency, by converting the binary data of image data outputted to the external device 10 such as a printer, etc., from a data memory 4 into the number of bit of multi-valued data by a data output means 8 in accordance with an area signal when an output is performed by an external device 2 such as a computer, etc., the image data inputted in a data input means such as an I/O 3, etc., is stored in the data memory 4 and the area signal discriminating binary data and multi- valued data is outputted by an area discrimination means 7 in accordance with area data showing the location of multi-valued data in the image data inputted in the same way.