Safe decontamination combustion system for forestry and agricultural waste including radiocesium, caused by Fukushima Nuclear Power Plant accident, was developed and the demonstration combustion test of contaminated forestry wastes was carried out. Thermal decomposition behaviors of contaminated biomass, cedar leaves, bark, sapwood and pine leaves were investigated by TG/DTA-FTIR. The laboratory scale combustion test of contaminated cedar bark (10 g) was carried out at 300 - 900 ^oC. Mass and radiocesium balances were evaluated, 30 - 35 % of radiocesium in bark evaporated by combustion above 500 ^oC. Gaseous radiocesium was trapped efficiently by water. The mass and radiocesium balances of demonstration test for contaminated forestry waste (6 kg) were evaluated. 95 % of radiocesium in contaminated forestry wastes was retrieved by combustion gas extraction and ash.