To control coating thickness of a lubricant with good accuracy and to improve durability by forming a magnetic layer on a nonmagnetic supporting body, applying a lubricant containing a material which gives fluorescent or absorption spectrum by irradiation of light on the surface side of the magnetic layer, and then detecting the layer.

A nonmagnetic supporting body 61 on which a magnetic layer and a carbon protecting film are formed is supplied from a supply roll 62 which rotates in the counterclockwise direction to a coating roll 63 which rotates in the clockwise direction, and the supporting body 61 is travelled around the coating roll 63 and wound up on a winding roll 64 which rotates in the clockwise direction. A coating liquid supply roll 65 is disposed to be in contact with the coating roll 63 to supply a lubricant 66 on the coating liquid supply roll 65, which is brought into contact with the nonmagnetic supporting body 61 travelling on the coating roll 63 so as to transfer and apply the lubricant 66. After coating, UV rays are emitted from a light source 72 to measure the amt. of the material which gives fluorescent spectrum in the lubricant. Thus, the coating thickness of the lubricant is measured and controlled to a const. value.