Portable H2 sensor was made by using mass spectrometer for the outside monitoring experiment: the leak test, the replacement test of gas pipe line, the combustion test, the explosion experiment, the H2 diffusion experiment and the recent issue of the exhaust gas of Fuel Cell Vehicle. In order to check the real time concentration of H2 in various conditions, even in the highly humid condition, the system volume of the sampling route was minimized with attaching the humidifier. Also to calibrate H2 concentration automatically, the specific concentration H2 small cylinder was mounted in the system. In the experiment, when H2 gas was introduced in the N2 flow or air in the tube or the high-pressure bottle, highly concentrated H2 phases were observed by this sensor without diffusion. This H2 sensor can provide the real time information of the hydrogen molecules and the clouds. The basic characterization of this sensor showed 0-100% H2 concentrations within 2ms. Our observation showed the size of the high concentration phase of H2 and the low concentration phase after mixing process. The mixed and unmixed H2, unintended concentration of cloud gas, the high speed small cluster of hydrogen molecules in purged gas were explored by this real time monitoring system.