An image data processing circuit includes a first circuit for calculating an average picture level value of the image data, a multiplier for multiplying a coefficient by the image data, a second circuit for calculating a first coefficient for the multiplier based upon the average picture level value, and a third circuit for calculating a second coefficient for the multiplier. The second coefficient is calculated so that the result of multiplying (1) image data having a value over an input range of a circuit to which data from the multiplier is inputted, and (2) the second coefficient falls in the input range. In addition, a selection circuit selects the coefficient to be used in the multiplier from the first coefficient calculated by the second circuit and the second coefficient calculated by the third circuit.