PURPOSE: To obtain the device which is lightweight and small sized and has superior mechanical strength and resonance resistance by composing a yoke member of a signal member having a counter magnetism-protective wall, yoke walls, and fitting flanges in one body.

CONSTITUTION: The yoke member 23 consists of the single member having the counter magnetism-protective wall 23a which is positioned in parallel to an optical disk 11, plural yoke wags 23b which cross the counter magnetism- protective wall 23a almost at right angles and extend in different directions from the peripheral edge part of the magnetism-protective wall 23a toward a carriage 13, and plural fitting flanges 23c which extend from the walls 23b onto the carriage 13 in one body. When this yoke member 23 is fixed to the carriage 13, the tips of the yoke walls 23b are connected by the counter magnetism-protective wall 23a, so the mechanical strength is superior. Consequently, there is no possibility of resonance even when the device is made thin.