A measurement flow path is formed in a detector that makes up a thermal type flow sensor. A temperature sensor is disposed on an upstream side, and a heater is disposed on a downstream side in a fluid flow direction of the measurement flow path. Furthermore, a heat transfer preventing wall is disposed between the temperature sensor and the heater. Preferably, a rectifying means such as an orifice or the like is disposed on the downstream side of the heat transfer preventing wall, and between the temperature sensor and the heater.