We have demonstrated terahertz (THz) passive imaging of room-temperature objects using a 4K-cryocooled THz photoconductive detector with background limited infrared performance (BLIP) at around 1.5-2.5THz. Images of a safety razor blade and a coin concealed in a plastic package or an envelope are successfully obtained with spatial resolutions of wavelength order using the THz passive imaging system. We have compared the measured THz intensity of several materials with emissivities calculated using the reported optical constants. The result shows that the THz intensity has a good linear relation to the emissivity, which means THz emissivity of an unknown material can be estimated at a room-temperature with the THz passive imaging system.