PURPOSE:To make it possible to detect fluctuations in the amount of float of a magnetic head with high accuracy and float said head in a highly stabilizing manner by using changes in an oscillating status of a semiconductor laser induced by the change in the span between a magnetic record medium and said semiconductor laser. CONSTITUTION:When an optical emitting end face is adapted to face a record medium 8 nearby and laser light reflected on the record medium is arranged to return to a semiconductor laser chip 6, the reflected light enters the laser where the oscillating condition of the laser is changed by a combined oscillating action between the semiconductor laser chip and the record medium. Therefore, the combined oscillating action between the semiconductor laser chip and the record medium is used so as to detect the changes in the span between both the laser chip and the record medium and it is also used to control the amount of float of a magnetic head 3. This construction makes it possible to float the magnetic head 3 in a highly stabilizing manner since the reflected light from the magnetic record medium is used.