An offshore wind turbine is comprised of a tower structure body configured with a tower and foundation, a rotor supported at the tower top, a power generator generating electricity by the rotation of the rotor, and a power converter converting electricity from the generator. A cavity is formed in the tower and foundation, and an internal space configured therewith is divided into upper and lower chambers by a bulkhead with first and second air vents between or in either of the tower and foundation. The converter is disposed in the upper chamber. An air circulating channel and air blower (e.g., air exhaust fan) are provided wherein air is circulated from the converter to the lower chamber through the first air vent, then to the converter through the second air vent. An air inlet is disposed at a lower level of an air outlet, or a partition member is provided therebetween.