There are three components which contain tritium in the air, i.e. water vapor, molecular hydrogen and methane. However, there is no tritium monitor, which is able to measure the concentration separately. In the light of it, we are developing a new monitor, which will be enable us to measure the tritium concentrations in two of these species, i.e. molecular hydrogen and methane. The specific activities of tritium in molecular hydrogen and in methane are very high. Then if we can collect the hydrogen and the methane, we will easily measure the tritium concentrations. We have conceptually designed a new tritium monitor and carried out several basic experiments. Based on the results of these experiments, we have estimated the performance of the monitor. For an hour counting, the monitor will be able to measure the tritium concentrations of molecular hydrogen and methane using about 4m3 air. Since the total performance of the monitor mainly depends on the capability of the proportional counter, we are developing an appropriate counter.