1. An image forming apparatus comprising: an image forming part, which forms an image in any one of a plurality of image forming modes with respectively different processing speeds; a density calibration control part, which performs image density control for said image forming part in a state where any one of the plurality of image forming modes is applied, wherein said density calibration control part makes a performing time interval for the image density control in a first image forming mode and a performing time interval for the image density control in a second image forming mode different from each other, and wherein if the image density control is performed in the first image forming mode, then said density calibration control part determines an image forming condition for the second image forming mode that has not been performed, based on results of the image density control; and a storing part, which stores a first table and a second table for determining an image forming condition that is used in the first image forming mode, and a third table and a fourth table for determining an image forming condition that is used in the second image forming mode; and a selecting part, which selects the first table and the fourth table when the image density control is performed in the first image forming mode, and selects the second table and the third table when the image density control is performed in the second image forming mode, wherein the first table is a table for determining an image forming condition for the first image forming mode, based on results of the image density control acquired in the first image forming mode, the second table is a table for determining an image forming condition for the first image forming mode, based on results of the image density control acquired in the second image forming mode, the third table is a table for determining an image forming condition for the second image forming mode, based on results of the image density control acquired in the second image forming mode, and the fourth table is a table for determining an image forming condition for the second image forming mode, based on results of the image density control acquired in the first image forming mode.