PURPOSE: To prevent a magnetic disk device, which uses data-erased sections as address marks, from erroneously detecting the address marks due to the noise of the data-erased sections.

CONSTITUTION: In a magnetic disk device which uses data-erased sections as address marks and detects the address marks by detecting data-erased sections of a specific period of time, an address mark detecting section 1 which monitors read data signals, is activated when the section 1 detects a data-erased section which is shorter than a specific address mark detecting period of time, and is cleared when the section 1 detects a data input is provided. In addition, a read data masking section 2 which prevents the erroneous detection of address marks due to erroneous read data pulses caused by the noise of the data-erased sections by inputting the read data signals and outputting an arbitrary number of read data pulses by masking the pulses immediately after the detecting section 1 is cleared.