Recently, soft proof, which can confirm the color reproduction of printed matter on a monitor, is coming into wide use in the field of graphic arts. However, there is a problem that the color appearing on the monitor is different from that of printed matter, even though the L*a*b* value of the monitor's white point has been adjusted to that of the paper's white measured by a spectroradiometer. According to our experiment, L*a*b* value of the monitor was more greenish blue in the case of LCD and bluish in the case of CRT than that of the paper, even though the color of the monitor was matched to that of the paper by visual calibration. In this paper, we assume that the cause of this phenomenon is bright line spectrums of the monitor and illuminations measured by a spectroradiometer were blurred by optical systems in the spectroradiometer. To solve the problem, we propose a method to correct this error by enhancing the bright line spectrums and report the effect of our method with two kinds of monitors (LCD and CRT) under three kinds of illuminations. After enhancing spectrums, L*a*b* values of the monitors were almost the same as that of the paper.