Global Change Observation Mission (GCOM) is a follow on mission of ADEOS, ADEOS2 and TRMM. It is under phase A study in NASDA (National Space Development Agency of Japan). GCOM is not a series of satellites but a mission and its concept is to continuously monitor geo-physical parameters which are critical to understand global change phenomena, especially phenomena related to climate change. Those parameters include, but not limited to, optical thickness of aerosols and clouds, water and energy fluxes, carbon fluxes, sink and source of greenhouse gases, etc. The measurements of geophysical parameters will continue more than 15 years after the launch of ADEOS2. The first generation satellites of GCOM after ADEOS2 is now composed of 3 satellites, i.e. GCOM-A1, GCOM-B1, and GPM core satellite. The target of GCOM-A1 is to monitor stratospheric and upper tropospheric greenhouse gases and ozone as well as ozone related constituents. The target of GCOM-B1 is to measure geophysical parameters which are uncertain in the today's climate models. GPM core satellite is a follow on of TRMM and the target of GPM core satellite is to measure precipitation. GPM mission is composed of GPM core satellite and 8 constellations of microwave radiometers. GCOM-A1 will carry 3 instruments, i.e. OPUS (Ozone and Pollution Ultra-violet Sounder), SOFIS (Solar Occultation Fourier Interferometric Sounder : ILAS follow on), and SWIFT (Stratospheric Wind Interferometer). GCOM-B1 will carry three core instruments, i.e. SGLI (GLI follow on), AMSR2 (AMSR follow on) and alpha-Scat (SeaW-inds follow on). Other instruments may be added based on an AO process. GPM core satellite will carry 2 instruments, i.e. DPR (Dual Precipitation Radar) and a microwave radiometer. The orbit of GCOM-B1 will be a sun synchronous orbit, which is almost the same as ADEOS2. The orbits of GCOM-A1 and GPM core satellite will be around 70 degree inclination orbit and the altitudes will be 700km and 400km, respectively. These 3 satellites are planned to be launched in 2008.