Embodiments of the present invention help to improve the cost, mountability, and reliability of a magnetic disk drive that regulates a fly height of a magnetic head slider correspondingly to the operating altitude of the drive by using an air pressure sensor and a heater. According to one embodiment, a preamplifier is a semiconductor chip formed by three-dimensionally laminating thin film circuits on a bulk silicon wafer. An air pressure sensor is integrally formed on the semiconductor chip through a semiconductor process. The air pressure sensor is configured to include an isolating cavity, a diaphragm that deflecting in response to a change in a pressure difference between the isolating cavity and the outside, and lower and upper electrodes and for detecting variation in electrostatic capacitance. The preamplifier including the air pressure sensor integrally formed therein is attached to a flexible printed cable (FPC) to oppose the FPC. A through-hole is provided in a portion opposite the air pressure sensor of the FPC to introduce air pressure in an operating environment into a diaphragm of the air pressure sensor.