Novel, safe, and highly effective agents for preventing insulin resistance and for preventing metabolic syndrome that contain astaxanthin and/or an ester thereof as an active component are provided. Even when a high-fat diet intake is continued, insulin resistance and subsequent hyperinsulinemia are suppressed by using the agent for preventing insulin resistance and the agent for preventing metabolic syndrome according to the present invention. Furthermore, suppression of fat degradation is thus inhibited and promotion of fat synthesis also is inhibited in adipose tissues, or glucose uptake into adipose tissues is inhibited, so that accumulation of fat within adipocytes is suppressed. Therefore, various diseases or symptoms having a relation to insulin resistance can be prevented or alleviated.