A PDE inhibitor and a therapeutic agent for a PDE-related disease of the present invention contain astaxanthin and/or esters thereof. The PDE inhibitor of the present invention is useful for treating various diseases exhibiting a relationship to PDEs, e.g., chronic obstructive pulmonary diseases, asthma, chronic artery obstruction, cardiovascular diseases, inflammatory diseases, allergic diseases, thrombosis, encephalopathy, hyperlipemia, and obesity.