Lavie (1995) proposed that under high perceptual load, attentional resources for task-irrelevant distractors are exhausted by the relevant task, which leads to a decrease in the distractor's interference. In contrast, Benoni & Tsal (2010) claimed that the decreasing interference effect by additional flankers is not due to perceptual load, but rather to early visual interference that depends on visual complexity. They called this effect “dilution”. However, other studies suggest that similarities in the processing of flankers and distractors are another important factor in the dilution effect (Miles, Yamaguchi, & Proctor, 2009; Roberts & Besner, 2005). The present study investigated the influence of processing similarity and perceptual load on suppression of interference. Interference was observed only when flanker-distractor similarity was low, regardless of visual complexity and perceptual load. These results suggest that the similarity is an important factor in interference suppression and that the exhaustion of resources is not solely determined by perceptual load. Our findings are consistent with the multiple resource theory (Wickens, 1980, 2002).