PURPOSE: To reduce the power consumption by setting an optimal write current easily and inexpensively and also to improve the quality by a stable read decoding in a recording and reproducing device which records information by means of supplying a prescribed write current to a head and reproduces with the head.

CONSTITUTION: An MPU 34 writes a measurement data read from a ROM 36 via an encoder 37 in a cylinder in a prescribed area of a magnetic disk by setting a prescribed number of write currents. And, a measurement signal which obtains a decoding margin from the MPU 34 at the time of reproducing is transmitted to a decoder 38 and the decoding margin such as a phase margin for every write current of read data is measured and a write current corresponding to the one among a prescribed number of write margins when saturation starts is set as an optimal write current and stored in the ROM 36.