an image sensor unit for transforming an optical signal into an electrical signal;

an Image Signal Processor (ISP) unit for receiving the electrical signal from the image sensor unit and outputting digitized image data;

a compression module containing memory, the compression module classifying image data according to frame when the image data are input, blocking each frame into a plurality of blocks having a predetermined size, performing Discrete Cosine Transform (DCT) on the individual blocks and then outputting DCT values related to the blocks;

a focus value calculating unit for receiving DCT coefficient values related to the plurality of blocks of a frame, quantizing the received DCT coefficient values using a fixed step size with respect to blocks in a window set region, and calculating focus values by integrating predetermined degrees of quantized DCT coefficient values; and

an auto focus Digital Signal Processor (DSP) for detecting a maximal focus value while moving a focus lens and performing auto focus adjustment when the focus values are input from the focus value calculating unit.