Control system for an internal combustion engine comprising a thermal flowmeter for measuring a fluid flow rate of a fluid flowing along a direction of flow comprising a heat resistor and a temperature sensor element that are formed on a thin film portion formed on a substrate, wherein the heat resistor is configured to be supplied with power from an engine control unit and the heat resistor is configured to have a temperature, when power is supplied, higher than a fluid temperature by a temperature increase ΔTh given in degrees centigrade, and a fluid flow rate is measured by using a value outputted from the temperature sensor element, a width of the heat resistor in the direction of flow of the fluid is in the range of 100 micrometers to 400 micrometers inclusive, and a maximum permissible power Phmax to be supplied to the heat resistor is set at 50 mW or less, wherein the thermal flow meter further includes the following configuration A or configuration B: - configuration A: wherein the heat resistor is made of impurity-doped polycrystalline silicon or single crystal silicon having a temperature coefficient of resistance of 1000 ppm/°C to 2000 ppm/°C, the temperature increase ΔTh is set so as to be between 80°C and 200°C inclusive, and a resistance value of the heat resistor is set so as to be between 360Ω and 540Ω inclusive; - configuration B: wherein the heat resistor is made of a metallic material having a temperature coefficient of resistance of 2000 ppm/°C to 4000 ppm/°C, the temperature increase ΔTh is set so as to be between 80°C and 300°C inclusive, and the resistance value of the heat resistor is set to be between 270Ω and 630Ω inclusive. characterized in that a length Lh and the temperature increase ΔTh of the heat resistor are set so as to satisfy a condition 1.4 ≤ ΔTh·Lh/Phmax ≤ 2.8, in order to ensure that a power consumption of the heat resistor does not exceed the maximum permissible power to be supplied to the heat resistor from the engine control unit, where Lh given in meters denotes the length of the heat resistor in a direction perpendicular to the direction of flow of the fluid and Phmax given in Watts denotes the maximum permissible power to be supplied to the heat resistor. - configuration A: wherein the heat resistor is made of impurity-doped polycrystalline silicon or single crystal silicon having a temperature coefficient of resistance of 1000 ppm/°C to 2000 ppm/°C, the temperature increase ΔTh is set so as to be between 80°C and 200°C inclusive, and a resistance value of the heat resistor is set so as to be between 360Ω and 540Ω inclusive; - configuration B: wherein the heat resistor is made of a metallic material having a temperature coefficient of resistance of 2000 ppm/°C to 4000 ppm/°C, the temperature increase ΔTh is set so as to be between 80°C and 300°C inclusive, and the resistance value of the heat resistor is set to be between 270Ω and 630Ω inclusive. characterized in that a length Lh and the temperature increase ΔTh of the heat resistor are set so as to satisfy a condition 1.4 ≤ ΔTh·Lh/Phmax ≤ 2.8, in order to ensure that a power consumption of the heat resistor does not exceed the maximum permissible power to be supplied to the heat resistor from the engine control unit, where Lh given in meters denotes the length of the heat resistor in a direction perpendicular to the direction of flow of the fluid and Phmax given in Watts denotes the maximum permissible power to be supplied to the heat resistor.