To realize a thermal gas-flow measuring instrument which can suppress the influence of pollution of a support of heating resistor on the output characteristics independent of the use environment. The first heating resistor (21) is placed in the gas to be measured. The temperature sensing resistor (24) which is positioned at upstream side or downstream side of the first heating resistor (21) generates signals relating to the flow rate. Between the first heating resistor (21) and the support (6) for supporting the first heating resistor (21), the second heating resistor (22) is positioned in a state of electrically insulated from the first heating resistor (21). The second heating resistor (22) suppresses the heat being transferred from the first heating resistor (21) to the support (2). The control circuit (31) controls the temperatures of the first heating resistor (21) and the second heating resistor (22) so as the operational temperature range of the second heating resistor (22) or the temperature range at the joint section between the first heating resistor (21) and the second heating resistor (22) to become at or above the temperature at which the water droplet evaporates to disappear by the film boiling.