PURPOSE: To reduce the time loss due to re-positioning, by decreasing the number of times of overrun production, through the constitution that the maximum interblocking gap length of command overrup can be varied.

CONSTITUTION: A maximum IBG setting section 1 setting a maximum interblocking gap (IBG) length, an IBG length count section 2 counting a tachogenerator pulse 9 from a capstan 6, a value B of the maximum IBG length set at the section 1, and a comparison section 3 comparing a value C of the IBG length counted at the IBG length count section 2, are provided. Further, a magnetic tape device is provided with an overrun detection section 4 detecting the overrun through a result (d) to be compared, and a running control section 5 making running control of a magnetic tape, and the maximum IBG length to be overrun can be variably set.