An image signal processor for processing an input image signal to output an output image signal includes an input unit receiving the input image signal, a noise removal unit removing noise from the input image signal, and a scaler reducing, maintaining or magnifying the image size of the input image signal. The scaler directly receives the input image signal from the input unit or the noise removal unit in response to a route control signal, reduces the image size of the input image signal when the image size of the input image signal is larger than the image size of the output image signal and stores the image signal with a reduced image size in a memory. The scaler maintains or magnifies the image size of the image signal stored in the memory and outputs the image signal with a maintained or magnified image size as the output image signal.