A subject-specific process of perceptual decision making is of importance to how the brain translates its interpretation of sensory information into behavior. In particular, a number of studies reported substantial variation across the observers' decision behavior, which may reflect different profiles of evidence accumulated by each individual. However, a detailed profile of perceptual integration has not yet been verified from human behavioral data. To address the issue, we precisely measured the time course of sensory integration, as the "sensory integration kernel" of subjects, using a coherence-varying motion discrimination task. We found that each subject has a distinct profile of sensory integration. We observed that kernel size (maximum sensory integration interval) is consistent within subjects, independent of external stimuli conditions. The observed kernel could accurately predict subject-specific perceptual behaviors and explain the inter-individual variation of observed behaviors. Surprisingly, the performance of most subjects did not improve in proportion to increased duration of the stimulus, but was maximized when the stimulus duration matched their kernel size. We also found that the observed kernel size was strongly correlated with the subject-specific perceptual characteristics for illusory motion. Our results suggest that perceptual decisions arise from intrinsic decision dynamics, and on individual timescales of sensory integration.