PURPOSE: To provide an image data managing system capable of preventing output image quality from being deteriorated by the display of enlargemet/ reduction, solving a problem to increase output processing time simultaneously, and attaining high quality and fast output.

CONSTITUTION: An electronic computer provided with a memory device in which image data is accumulated is equipped with an image division processing mechanism 2 which divides an image 1 into rectangular areas, a picture element thinning processing mechanism 5 which generates a new image by thinning the picture elements of the image to the outputtable number of picture elements, a hierarchy switching mechanism 7 which selects the optimum image unit when the image is outputted at prescribed magnification, an output conversion mechanism 8 which performs the enlargement/reduction of the image with prescribed magnification, and an output device 9 which performs processing by using the image accumulated in hierarchical fashion in the electronic computer selectively when the output after enlargement/reduction processing is performed is issued.