High angular resolution astronomical observation in far-infrared better than one milli-arcsecond will open a new field of astronomy, such as imaging inner region of protoplanetary disk and broad line region of active galactic nuclei. We propose to apply intensity interferometry in far-infrared wavelengths for aperture synthesis imaging. Intensity interferometry is known by the Hanbury-Brown and Twiss experiment in 1950’s, but aperture synthesis imaging was not possible because of missing phase information. Modern fast electronics enable a usage of fast photon detectors, and delay time measurements of photon bunches enable aperture synthesis imaging. There are several merits using intensity interferometry in far-infrared wavelengths. First, high sensitivity direct detectors can be used to realize background limited observations from space, not limited by quantum noise of heterodyne receivers. Second photon signal can be recorded, and correlation analysis can be made in post data analysis, and there is no need for beam combining optics. These lead to flexible telescope configuration in space; number of telescopes is not limited, attitude control of satellite can be independent, and baseline configuration is flexible. We have demonstrated the delay time measurements in microwave, some simulation studies for aperture synthesis imaging were made and developing fast photon detectors in terahertz frequencies. Experimental setup for intensity interferometry is being prepared. Prospects of far-infrared intensity interferometers on ground and in space are discussed.