It is determined whether an image before pixel mixture has a high frequency. In a partial image obtained by an imaging device having a Bayer array, a Gr pixel which indicates a green component and is arranged in the same row as an R pixel indicating a red component is distinguished from a Gb pixel which indicates a green component and is arranged in the same row as a B pixel indicating a blue component. The Gr pixels and the Gb pixels which are distinguished from each other are separately mixed. When an image before pixel mixture has a high-frequency component, the level of the Gr pixel and the level of the Gb pixel are different from each other after pixel mixture. It is determined whether the image before pixel mixture has a high-frequency component on the basis of the level of the Gr pixel and the level of the Gb pixel after pixel mixture.