This study examines if and how application of cogeneration to largescale housing complex in Tokyo metropolitan area can be made. Using results from detailed energy consumption survey, 170 cogeneration model systems were created, and primary energy conservation ratio and utility cost saving ratio were calculated through hourly simulation. Following results were obtained : (1)20 % energy conservation and 10 % economic benefit can be achieved with either gas or diesel engine cogeneration system when it is applied to the entire complex rather than to restricted sections like utility plants. (2) Both benefit can also be maximized when electricity is exclusively provided by cogeneration.