An image processing apparatus comprising: a reflection characteristic detector configured to detect reflective characteristics of respective output images formed at different points in time on respective print media in accordance with input image data; and a gradation characteristic correction unit configured to correct gradation characteristics of the input image data such that the reflection characteristics of the respective output images on the respective print media detected by the reflection characteristic detector match, wherein the reflection characteristic detector comprises a gradation control parameter calculation unit configured to calculate gradation control parameters for controlling gradation characteristics of a basic color of output images formed on the print media, based on reflection characteristics of the output images formed on the print media, and the gradation characteristics correction unit is configured to correct the gradation characteristics of the input image data based on a result of comparison between a set of first gradation control parameters and a set of second gradation control parameters, the set of first gradation control parameters being a gradation control parameter calculated by the gradation control parameter calculation unit and serving as a standard value, the set of second gradation control parameters being newly calculated by the gradation control parameter calculation unit, the image processing apparatus further comprising a determining unit configured to determine a validity of the gradation control parameter calculated by the gradation control parameter calculation unit for use in correction of gradation characteristics of the input image data in the gradation characteristic correction unit based on a required information amount; and a correction unit configured to correct a value of the gradation control parameter based on the validity, characterized in that the determining unit is configured to perform the determining on a first plurality of pages as one set, and the correction unit is configured to perform the correcting on a second plurality of pages as a second set. a reflection characteristic detector configured to detect reflective characteristics of respective output images formed at different points in time on respective print media in accordance with input image data; and a gradation characteristic correction unit configured to correct gradation characteristics of the input image data such that the reflection characteristics of the respective output images on the respective print media detected by the reflection characteristic detector match, wherein the reflection characteristic detector comprises a gradation control parameter calculation unit configured to calculate gradation control parameters for controlling gradation characteristics of a basic color of output images formed on the print media, based on reflection characteristics of the output images formed on the print media, and the gradation characteristics correction unit is configured to correct the gradation characteristics of the input image data based on a result of comparison between a set of first gradation control parameters and a set of second gradation control parameters, the set of first gradation control parameters being a gradation control parameter calculated by the gradation control parameter calculation unit and serving as a standard value, the set of second gradation control parameters being newly calculated by the gradation control parameter calculation unit, the image processing apparatus further comprising a determining unit configured to determine a validity of the gradation control parameter calculated by the gradation control parameter calculation unit for use in correction of gradation characteristics of the input image data in the gradation characteristic correction unit based on a required information amount; and a correction unit configured to correct a value of the gradation control parameter based on the validity, characterized in that the determining unit is configured to perform the determining on a first plurality of pages as one set, and the correction unit is configured to perform the correcting on a second plurality of pages as a second set.