A bonnet type stear- generating apparatus includes ultrasonic atomizer means (30,38) for-generating a mist at room temperature and an electric heater (13) disposed in a passageway (47,12,9) from the atomizer means (30,38) to a bonnet (2) for heating the mist to provide steam which is delivered to the interior of the bonnet (2) wherein a customer's head is inserted for hair treatment. Preferably, the steam is ejected upwardly from the entire periphery of the bonnet (2). Control circuitry incorporating a microprocessor (105) senses and regulates the temperature of the steam with the aid of a temperature sensor (3). The temperature of the steam and the duration of operation of the apparatus are easily selectable and adjustable by the use of manual switches (62 to 69). The apparatus can be used in another mode to deliver hot air, instead of steam, to the bonnet (2).