To clarify the four processing stages in the Stroop task (input, decision, response selection, and output), and to examine on which stage or stages the interference occurs, seven undergraduates were used as subjects in a color-naming task with color names, words strongly associated with color, words weakly associated with color, a neutral word, and "X X X" as control stimulus. Color-patch pieces were also used to measure the operant level of reaction time. Main results were as follows: 1. On the congruent trials, the response time was shorter for color names than for the neutral word, control word, and words with weak association. 2. On the incongruent trials, the response time was longer for color names than for the neutral word, control word, and words with weak and strong association. Moreover, the interaction between pre-exposure duration and word conditions was significant; the difference of response time due to word conditions as significant when the pre-exposure duration was short, while it was not significant when the duration was long. 3. The neutral word and the control word gave similar response time, which was longer than that for color-patch pieces. From these results it is concluded that the interference in the Stroop task is most likely to occur on the decision and the output stages.