A condensed water drain structure includes an air conditioning case having an evaporator and a drain for draining condensed water from inside the air case to the exterior of a vehicle body. A drain cap mounted on the drain has a head with a hole for draining condensed water. The hole is shaped as a slot having a height difference toward the air conditioning case in an axial direction of the head. The condensed water is smoothly drained even though there is a narrow space between a plate in the case and structure external to the case, to prevent vehicle exhaust gas or smoke from entering of the vehicle body.