In far-infrared astronomical observation, angular resolution have been limited by
diameters of cryogenic telescopes. Far-infrared interferometry is the next step after
SPICA and Origin Space Telescope.  To make use of great sensitivity of cryogenic
detectors, ultimate choice is photon counting detectors whose sensitivity is only
limited by background photon noise. Further, intensity interferometry can make use of
such photon counting detectors for aperture synthesis imaging when fast readout
electronics are employed together with accurate attitude and timing control, and fast
data recording and transmission.  I will start discussing astronomical interest of high
angular resolution far-infrared observations, followed by technical details and current
experimental status toward the future far-infrared interferometry in space.