A delay locked loop that applies an amount of delay to an external clock signal to generate multiple delayed signals. When the recording linear density that has to be attained to ensure a prescribed capacity cannot be attained by the head, the recording linear density that has to be attained by the head is reduced and the prescribed capacity is realized by expanding the data recording area. A computation control section calculates the flow velocity distribution. The controller can be configured to compute a first threshold level corresponding to background noise and a second threshold level corresponding to sensor noise, and to compute the report threshold value from the second threshold.