A magnetic disk device includes a magnetic head assembly, which includes magnetic heads, support plates, and a flexible wiring substrate that are integrally formed. The magnetic heads include heating elements for making head element parts protrude toward magnetic disks by thermal expansion, the support plate supports the magnetic head, the flexible wiring substrate is provided along the support plate and electrically connects the magnetic heads to a circuit system, and the magnetic head assembly and the magnetic disks are assembled in a case. The magnetic disk device includes a sensor and a floating distance control circuit. The sensor detects at least one of atmospheric pressure, temperature, and humidity in the case. The floating distance control circuit increases or decreases current supplied to the heating element on the basis of the output of the sensor and controls the floating distance of the magnetic head so that the floating distance is constant. While being received in a package made of high temperature co-fired ceramic, the sensor is mounted on a circuit mounting surface of the flexible wiring substrate in the case.