We propose a bias operation method with high stability using a high extinction-ratio optical modulator. Even in the high extinction-ratio optical modulator, there is instability of the bias fluctuation under a null-point bias operation, in general. We set the bias point at a slope for improvement of tracking performance without significant spurious emissions. When the bias voltage is changed in operation by bias drifts of the modulator, slope-set bias operation can easily provide direction of the voltage change, and finally, can realize quick restoration of the bias condition without unwanted bias changes. Numerical and experimental analyses are discussed in the study with a discussion of nonlinear distortion effect at bias conditions.