PURPOSE: To measure the contact force of a magnetic head highly accurately by minutely displacing the rotary magnetic head in correspondence with the contact force, and optically measuring the amount of displacement.

CONSTITUTION: A rotary magnetic head 1 is supported by a head supporting means 5 and can be displaced minutely in the direction of A in correspondence with the contact force with a magnetic tape 3. As the supporting means 5, a head assembly 4, which has, e.g., an L shaped cut-in part 7 and is formed so that the assembly is displaced in the direction A in correspondence with the contact force with the tape 3, is used. The amount of displacement of the head 1 is measured by a photoelectric-conversion measuring means 6. The measuring means 6 is provided in a rotary cylinder 2 so that the means 6 is rotated as a unitary body with the head 1. In this constitution, the contact force of the magnetic head can be measured highly accurately.