Space debris smaller than 1 mm in size still have enough energy to cause a fatal damage on a spacecraft, but such tiny debris cannot be followed or tracked from the ground. Therefore, IDEA the project for In-situ Debris Environmental Awareness, which aims to detect sub-millimeter-size debris using a group of micro satellites, has been initiated at Kyushu University. The IDEA project aims to construct the dynamic environmental model that provides a better definition of the current orbital debris environment. This study proposes data assimilation method to estimate the environment of sub-millimeter-size debris using in-situ measurement data. This paper also verifies the proposed model by simulating the measurement and the estimation. The simulation demonstrates that the estimation model can determine the orbital plane of the detected debris approximately.