To provide a windmill excellent in cost performance, serviceability and reliability with a simple structure and capable of cooling a heat generating part in a hub effectively and efficiently.

This windmill is provided with an intake port 2 and a duct 3 guiding to the heat generating parts 4a, 4b on a hub 31, and has a structure establishing communication between a space in the hub where the heat generating parts are arranged and discharge ports 5a, 5b in a blade tip via a space in a blade. Pressure difference between a section near the intake port and a section near the discharge port is generated by rotation of a rotor, outside air is sucked from the intake port and flows in the duct and is discharged from an outlet 3a of the duct, and the outside air impinges on the heat generating part in the hub, is heated by absorbing heat from the heat generating part, enters each blade and is discharged to outside of the windmill 1 from each discharge port. The heat generating parts in the hub are cooled by this air current.