[en] Described are the outline of Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) heavy ion microbeam (MB) system of Japan Atomic Energy Agency, possible to irradiate at a unicellular level, biological study results obtained with the system and the MBs present outside Japan. TIARA has developed MB system of the minimal diameter of several micro-meters. Such MB can irradiate one target cell, which enables to analyze the direct intracellular and indirect (bystander) radiation effects, differing from those past studied by radiation like X-ray (random irradiation to cell population) which has only given the effects as an average in the cell population. TIARA now has such MBs as ⁴He²⁺ (diameter 40-250 micro-meter), ¹²C⁵⁺ (20-250), ²⁰Ne⁸⁺ (20-250), ²⁰Ne⁷⁺ (5-250), ⁴⁰Ar¹⁴⁺ (5-250) and ⁴⁰Ar¹³⁺ (5-250), which are produced in azimuthally varying field cyclotron. These MBs have been used as a probe for analysis of biological functions in the development and differentiation of a silkworm, and in the gravitropism and hydrotropism of root cells of a plant; and for analysis of bystander effects in Chinese hamster ovary (CHO)-K1 cells, human fibroblasts, germ cells of a nematode C. elegans, cultured tobacco BY-2 cell protoplasts, and mouse skeletal muscle cells. At present, facilities having MBs of ions heavier than He, other than TIARA, are only German GSI, Gesellschaft fuer Schwerionenforschung, and Technische Universitaet Muenchen. Progress of MB technology will be further promising with its wider application aiming a novel cancer therapy, elucidation of biological effects produced by high linear energy transfer (LET) radiation like a cosmic ray, better breeding technology, and gene biotechnology. (R.T.)