1. An image processor, comprising an image processing section which performs a blurring processing on an image which is acquired via a lens system and includes an image of a main subject and an image of a background, the image processing section including: a reduction rate determiner part which determines a reduction rate of the background image based on at least one of a focal length of the lens system, a distance between the main subject image and the background image, a level of the blurring processing determined by a user's instruction, and an image size of the background image; an image reduction part which reduces the background image at the reduction rate determined by the reduction rate determiner part; a spatial filter processing part which performs a spatial filter processing on the background image reduced by the image reduction part; and an image enlargement part which enlarges the background image processed by the spatial filter processing part at an enlargement rate as an inverse of the reduction rate, wherein a subject of the blurring processing of the image processing section is the background image.