By combining one or more drugs selected from the group consisting of sedative antidepressants and antihistamines with (S)—N-[2-(1,6,7,8-tetrahydro-2H-indeno[5,4-b]furan-8-yl)ethyl]propionamide, there is provided a prophylactic or therapeutic agent for sleep disorder that induces natural sleep, shortens sleep latency, increases deep sleep, is excellent in maintaining sleep, and exerts to attain appropriate sleep duration.