It is an object of the present invention to improve a measurement accuracy of a thermal flowmeter. A circuit package 401 is such that a measurement surface 430 and a backside of measurement surface 431 of a rear surface thereof are located in a bypass passage, and the bypass passage is configured to allow a measurement target gas 30 to flow upon dividing the measurement target gas 30 into a flow path 386 at a side of the measurement surface 430 of the circuit package and a flow path 387 at a side of the backside of measurement surface 431 of a rear surface of the measurement surface 430, and an inflow-side end surface of the circuit package for dividing the measurement target gas 30 has different shapes at the side of the measurement surface and at the side of the backside of measurement surface. The inflow-side end surface of the circuit package for dividing the measurement target gas 30 is formed with a reference line 700 dividing the measurement target gas 30, and an end surface 701a at the side of the measurement surface with respect to the reference line and an end surface 701b at the side of the backside of measurement surface with respect to the reference line are formed to be asymmetrical.