An image processing device for generating display image data for displaying a main image and a sub image simultaneously or in a switching manner, the image processing device comprising: a main image data input means for receiving input of main image data representing the main image; a sub image data input means for receiving input of sub image data representing the sub image; a display mode acquisition means for acquiring display mode information indicating a display mode for displaying the main image or the sub image, or both the main image and the sub image simultaneously or in a switching manner; a first feature amount calculation means configured to receive the main image data from the main image data input means and use part or all of the main image represented by the received main image data to calculate a feature amount regarding image quality of the main image data; a second feature amount calculation means configured to receive the sub image data input to the sub image data input means or resized sub image data obtained by resizing the sub image data in accordance with the display mode information acquired by the display mode acquisition means, and use part or all of the image represented by the received image data to calculate a feature amount regarding the image quality of the received image data; an image quality adjustment means for performing image quality adjustment on the resized sub image data based on the feature amount calculated by the first feature amount calculation means and the feature amount calculated by the second feature amount calculation means to generate adjusted sub image data; and a display image data generation means for generating the display image data from the main image data input to the main image data input means and the adjusted sub image data generated by the image quality adjustment means, in accordance with the display mode information acquired by the display mode acquisition means, wherein the image quality adjustment means is configured to perform the image quality adjustment so that a difference between a feature amount of the adjusted sub image data and the feature amount calculated by the first feature amount calculation means is smaller than a difference between the feature amount calculated by the second feature amount calculation means and the feature amount calculated by the first feature amount calculation means, and the sub image data include image data of a plurality of content items arranged in time series, the image quality adjustment means being configured such that, when the image quality adjustment means performs the image quality adjustment for each of the content items, the image quality adjustment means updates the feature amount regarding image quality of the main image data used in the image quality adjustment, at the timing of change of the content item subjected to the image quality adjustment. a main image data input means for receiving input of main image data representing the main image; a sub image data input means for receiving input of sub image data representing the sub image; a display mode acquisition means for acquiring display mode information indicating a display mode for displaying the main image or the sub image, or both the main image and the sub image simultaneously or in a switching manner; a first feature amount calculation means configured to receive the main image data from the main image data input means and use part or all of the main image represented by the received main image data to calculate a feature amount regarding image quality of the main image data; a second feature amount calculation means configured to receive the sub image data input to the sub image data input means or resized sub image data obtained by resizing the sub image data in accordance with the display mode information acquired by the display mode acquisition means, and use part or all of the image represented by the received image data to calculate a feature amount regarding the image quality of the received image data; an image quality adjustment means for performing image quality adjustment on the resized sub image data based on the feature amount calculated by the first feature amount calculation means and the feature amount calculated by the second feature amount calculation means to generate adjusted sub image data; and a display image data generation means for generating the display image data from the main image data input to the main image data input means and the adjusted sub image data generated by the image quality adjustment means, in accordance with the display mode information acquired by the display mode acquisition means, wherein the image quality adjustment means is configured to perform the image quality adjustment so that a difference between a feature amount of the adjusted sub image data and the feature amount calculated by the first feature amount calculation means is smaller than a difference between the feature amount calculated by the second feature amount calculation means and the feature amount calculated by the first feature amount calculation means, and the sub image data include image data of a plurality of content items arranged in time series, the image quality adjustment means being configured such that, when the image quality adjustment means performs the image quality adjustment for each of the content items, the image quality adjustment means updates the feature amount regarding image quality of the main image data used in the image quality adjustment, at the timing of change of the content item subjected to the image quality adjustment.