A fan includes sloping surfaces arranged at an inner peripheral surface of an air channel portion that are shaped such that an air passage is enlarged in a cross sectional area in a direction normal or substantially normal to a central axis. The inner peripheral surface of the air channel portion also includes a straight surface at which area the distance between the central axis and the inner peripheral surface of the air channel portion is substantially constant. Also, the straight surface of the air channel portion includes a plurality of slits each penetrating the air channel portion.