From different aspects than the frequency response analysis of the visual system, we investigated characteristics of the nonlinear response of the visual system to flickering stimuli in low temporal frequency domain. We measured increment threshold curves (masking functions) for 2 Hz flicker of rectangular and sawtooth waveforms, using six and three subjects, respectively. When the amplitude of the flicker was at a supraliminal level, the increment threshold curve had on-transients for the rapid onset larger than off-transients for the rapid offset of the flicker wave. However, around a liminal amplitude level off-transients were more prominent. This result suggest that at low temporal frequencies (2 Hz) the temporal contrast sensitivity may be determined mainly by the 'off-responses' in the visual system. Moreover, we found the similar changes of increment threshold curves to the flicker stimuli for a single rectangular stimulus, which means that the results as above mentioned are not specific to the periodical stimuli.