In a flow sensor, detecting elements are provided at a sub-passage. A sub-passage wall contains a hole to drain accumulated liquid. A protrusion arranged close to the hole on an external surface generates dynamic pressure on the opening in response to external flow. Alternatively, a protrusion upstream from the hole on the inner wall surface produces a separation flow area for separating the flow from the internal surface near the hole, whereby pressure in the separation area is reduced and almost the same pressure differences on the internal and external surfaces openings result. This reduces leakage from the hole and changes in the distribution flow in the sub-passage between cases where the hole is blocked and not blocked, thus minimizing flow measurement errors. Structure may be provided close to the external surface hole opening to prevent a liquid film or drop from being formed on the opening by surface tension.