Thermal flow meter with an asymmetrical inflow-side of circuit package

A thermal flow meter including a bypass passage for flowing a measurement target gas received from a main passage, an air flow sensing portion for measuring a flow rate of the measurement target gas by performing heat transfer with the measurement target gas flowing through the bypass passage, and a circuit package having the air flow sensing portion,

• wherein the circuit package is such that a measurement surface and a backside of a measurement surface of the rear surface thereof are located in the bypass passage,
• the bypass passage is configured to allow the measurement target gas to flow therein dividing the measurement target gas into a flow path at a side of the measurement surface of the circuit package and a flow path at a side of the backside of the measurement surface of a rear surface thereof,
• an inflow-side end surface of the circuit package for dividing the measurement target gas has different shapes at the side of the measurement surface and at the side of the backside of the measurement surface, and
• a metal substrate for fixing the air flow sensing portion is molded by a molding process, wherein the metal substrate is exposed on the backside of the measurement surface.