An integrated circuit in a hard disk drive contains a resistance measurement circuit for a magnetoresistive head. The relationship between the amount of a resistance change in the magnetoresistive head and the amount of a temperature rise, the relationship between the amount of a temperature rise and the average amount of magnetoresistive head protrusion, and a permissible temperature rise value for magnetoresistive head life optimization are entered in a PC. The amount of a resistance change is calculated from a magnetoresistive head's initial resistance value and a resistance value prevailing after power application to a heater. The amount of a temperature rise is determined from the amount of a resistance change. The amount of protrusion is determined from the amount of a temperature rise. If a required protrusion amount is not reached by the amount of protrusion, the amount of power application is increased. If the required protrusion amount is reached by the amount of protrusion, the prevailing amount of power application is regarded as a heater's initial power application amount for shipment and stored in an MPU.