PURPOSE: To improve S/N ratio by forming a rhombic deposition type magnetic layer which contains Co, Cr and O or Co, Ni, Cr and O as main components and Mg of a specific % of number of atoms on a supporting body.

CONSTITUTION: A deposition equipment is evacuated by an exhausting system 9 and a vacuum chamber 8 is kept at a required degree of vacuum. A supporting body 5 of tape-shape made of polyethylene terephthalate is supplied by a pay-off roll 6 and supported by a cylindrical cooling can 1 and guided to a take-up roll 7. Co-Ni-Cr-Mg alloy or Co-Cr-Mg alloy is charged to an evaporating source 2. A mask 3 determines a minimum slant incident angle θmin. Oxygen is taken into the magnetic layer by exposing the layer to an oxygen atmosphere after deposition is completed. A rhombic deposition type magnetic layer which contains Co, Cr and O or Co, Ni, Cr and O as main components and Mg of 0.05W3% of number of atoms is formed on the supporting body and improved S/N ratio is obtained.