PURPOSE: To record an intermediate image with high image quality by obtaining a conversion characteristic of an analog screen generator and revising image data conversion information of a data conversion means so that the conversion characteristic is coincident with an ideal stored pulse width conversion characteristic.

CONSTITUTION: Input image data are given to lookup tables LUTs 2, 3 by using channels 1, 2 and an output from them is given to analog screen generators ASG4, ASG5. Then number of simulated image data from the LUTs 2, 3 is 0 to 255, e.g. 16 points, e.g. at an interval of 16, that is, 0, 15, 31-255, they are given to 1st and 2nd generators ASG 4, 5, in which the data are converted into a pulse width modulation signal. Then the pulse width of the pulse width modulation signal from the 1st and 2nd generators ASG 4, 5 is detected by using a detector 7 and the pulse width is read by a controller 8. Furthermore, the switching of the channel 2 is implemented similarly to make image data from the controller 8 complete and the result is outputted as gradation characteristic data B1, B2.