In psychophysical scaling, a subjective rating in the current trial is tend to be biased by preceding trials (sequential effect). Here, we investigated the factor determining the sequential effect: physical stimulus, evaluation process, or response in the preceding trials. Participants observed disks with different luminance values and rated the perceived brightness of the disk on a 9-point scale. When participants were required to evaluate and response to the 1-back trials, the current rating was assimilated toward 1-back trials. On the other hand, when participants were not required to evaluate the 1-back stimulus, the current rating was not affected by 1-back trials, independent of whether the response to the 1-back stimulus was required or not. These results suggest that the sequential effect was due to evaluation process but not to stimulus or response in the preceding trials.