There are provided an image processing apparatus, an endoscope apparatus, and a color balance adjusting method capable of obtaining a good observation image. An image processing apparatus of the invention includes: an image signal input unit for inputting an image signal of a subject image picked up by an image pickup unit, the subject image including at least an image of a color balance adjustment tool; a control unit for calculating a value of coefficient to change brightness value of the image signal, based on a correction value to correct characteristic variation of the color balance adjustment tool, the correction value being shown in a correction value display portion included by the color balance adjustment tool, and a brightness value of the image of the color balance adjustment tool; and a color balance process unit for performing color balance adjustment by changing the brightness value of the image signal based on the coefficient value.