Global Change Observation Mission (GCOM) is a follow on mission of ADEOS and ADEOS2. It is under phase A study in NASDA (National Space Development Agency of Japan). GCOM is not satellites but a mission and its concept is to continuously monitor geophysical parameters which are critical to understand global change phenomena, especially phenomena related to climate change and ozone depletion. The first generation of GCOM is now composed of 2 satellites, i.e. GCOM- A1 and GCOM-B1. The target of GCOM-A1 is to monitor greenhouse gases distribution and ozone as well as ozone related constituents from oblique orbit. It is now planned to carry two core instruments, i.e. ILAS2 F/O and ODUS. ILAS2 F/O is a sun occultation sensor using a Fourier transform spectrometer and measures vertical distribution of atmospheric constituents. ODUS is an ultraviolet to visible grating spectrometer and measures total ozone and aerosols. The target of GCOM-B1 is to measure geophysical parameters which are uncertain in today's climate models. Those parameters include, but not limited to, optical thickness of aerosols and clouds, thermal fluxes, carbon fluxes, sink and source of greenhouse gases, etc. GCOM-B1 will carry four core instruments, i.e. SGLI (GLI follow on), AMSR2 (AMSR follow on), alpha-Scat (SeaWinds follow on), and APOLDER (POLDER follow on). Another candidate instrument is ATRAS (IMG follow on). The orbit of GCOM-B1 will be a sun synchronous orbit, which is almost the same as ADEOS2.GCOM-A1 is planned to be launched in Feb. 2005 while GCOM-B1 is planned to be launched in Aug. 2005.