A flow detecting device is provided to a fluid passage through which a main flow of fluid passes. A sensor body has a bypass passage through which a bypass flow passes from the main flow. The bypass passage has a bent portion, which is located midway through the bypass passage, and an outflow passage, which is located downstream of the bent portion. The flow direction of the bypass flow changes at the bent portion toward the outflow passage through the bypass passage. A heating element is arranged in the bypass passage. The heating element generates heat by being supplied with electricity for detecting a flow amount of fluid. The sensor body has a side surface defining an opening through which dynamic pressure caused by counterflow is released to an outside of the outflow passage when fluid causes counterflow in the fluid passage in a direction opposite to a flow direction of the main flow.