An image capturing unit 1 acquires a plurality of images being temporally continuous. Based on external operations from a key input unit 4, a control unit 9 selects an image serving as reference of synthesis, from the plurality of images acquired by the image capturing unit 1. An image processing unit 10 compares the reference image selected by the key input unit 4 with other images acquired by the image capturing unit 1, and detects an image with a difference value being higher than a predetermined threshold value as a result of such comparison. The image processing unit 10 sets a synthetic rate, based on the image thus detected. A synthesis unit 11 generates a single image by executing synthesis of the plurality of acquired images including the image serving as reference, by using the synthetic rate that is set by the image processing unit 10.