PURPOSE: To reduce the error of measurement and to enhance the reliability as well as to facilitate the measurement, by bringing a thin film into abutment on the outer circumference face of a cylinder at a prescribed angle and calculating the degree of projection of a magnetic head from the number of contour stripes according to the swelling of the surface on the thin film.

CONSTITUTION: A tape 14 is brought into abutment at a prescribed angle on the circumference face 4 of a rotary cylinder 3 and a fixed cylinder 13 and over these cylinders at a cylinder part where a magnetic head 1 is assembled. At the same time, a grating plate 12 having numbers of slits cut longitudinally is set in parallel with and in the vicinity of the bus bar of the face 4. Then the face of the tape 14 which is opposite to the contact face to the head 1 is irradiated by a light source 15 and via the plate 12. Moire fringes are observed when the above-mentioned face is viewed through a magnifier 16 and from the side of an arrow III. The degree of projection is made visible and can easily be measured for the head 1 assembled in the cylinder.