A new thoron ((220)Rn) experimental room (TER) was established at National Institute of Radiological Sciences in 2011 to respond to the appearance of new passive devices for (220)Rn and its progeny ((212)Pb, (212)Bi) measurement. The room volume is 21.7 m(3) with a low air exchange rate, 0.016 h(-1). Environmental parameters, such as temperature, relative and absolute humidities, wind speed, aerosol distribution and concentration, are monitored, but not controlled. Commercially available lantern mantles (500 pcs), which can generate maximum concentrations of 6000 Bq m(-3) of (220)Rn and 2000 Bq m(-3) of its progeny, are used as a (220)Rn source. Based on preliminary results from the TER, stable and homogenised concentrations of (220)Rn and its progeny can be ensured; therefore, in the near future the room will be used for testing measuring instruments.