We have developed a new continuous monitoring system for volcanic gas chemistry and set up this system at 3 km north from the summit of Izu-Oshima volcano. For continuous monitoring of dry gas, we use three types of dehydration methods to remove water vapor. CO₂ and SO₂ contents were measured by IR sensors, O₂ by a zirconia sensor and H₂ by a semiconductor sensor. Our system has been in operation more than 6 months without significant troubles. The dry volcanic gas is a mixture of CO₂ of magmatic origin and air component. Temporal variation in chemical composition of the volcanic gas is related with that in atmospheric pressure. The CO₂ content is inversely-correlated and O₂ is correlated to atmospheric pressure with 8 hours' delay. In contrast, H₂ content increases intermittently up to 1000ppm 1 hour after sudden drop of atmospheric pressure. The N₂/O₂ ratio is higher than the air ratio (3.8), and changes in a similar manner to CO₂. This suggests that a part of N₂ is of magmatic origin like CO₂.