PURPOSE: To contrive improvement of sensitivity by forming a ferrite film consisting of the mixture of ferrite powder and a binder of prescribed quantity, and forming a radiation-ray-absorbing material film consisting of radiation-ray- absorbing material on one surface of the above-mentioned ferrite film.

CONSTITUTION: After a substrate C has been coated with the mixture of ferrite powder of 50 to 90wt.% and a paste-like binder of 10 to 50wt.%, a ferrite film A is formed by drying up the coating for 30 minutes to several hours. Then, one face of the ferrite film A is coated with liquid or paste-like radiation-ray absorbing material, and a radiation-ray absorbing film B is formed. A sample is arranged on the magnetic field which is generated by a magnetic-field generator 100, the permeability of a photo-sensitive magnetic body is changed by changing the illuminance of a light, detected by a magnetic head 101, and the amount of change of output is computed. Besides, the sensitivity of the ferrite powder is deteriorated when with 50wt.% or less, and the adhesive force with the substrate C becomes weak when the content is 90wt.% or more.