A unique system that uses the total reactivity with ozone (RO3) has been developed for measuring biogenic volatile organic compounds (BVOCs) emitted from vegetation into the atmosphere. However, the sensitivity of the RO3 analyzer was insufficient for measuring the atmosphere in the forests. Recently, the sensitivity of the ozone monitor in the analyzer has been improved. In this study, further improvement of the analyzer was explored. As a result of reduction of noises in the detector, the detection limit of RO3 analyzer reached 0.2 x 10-4 s-1 (S/N = 3, 60-s average). After the improvement, a practical test for measuring RO3 in the forest was conducted at Tokorozawa in summer. Consequently, it was experimentally confirmed that coexisting NO can be critical as interference of measurement. This issue remains to be solved. Additionally, in order to realize a versatile procedure for measuring RO3 in the forests, combination of sampling and RO3 analysis was challenged. At present, the signal of BVOCs as RO3 was preliminarily captured. It was indicated that the sampling procedure be promising.