The characteristics of a thermo tube type thermal gas flowmeter were analyzed and experimental results were described. The output of this flowmeter is temperature difference between two points along an electrically heated capillary. An analytical expression of its outputs was derived assuming a simple flow condition. An experimental prototype was constructed. The temperature distribution was measured and the outputs were obtained for three kinds of gases: nitrogen, carbon dioxide, and argon. Those outputs were compared with theoretical ones, and those agree well in the case of small flow rate. The relationship among temperature difference, mass flow rate, and specific heat of gas was discussed.