PURPOSE: To simply mount a head arm to a base plate by forming a recessed part and a notch part in the range of a bottom plate of the base plate where the head arm moves.

CONSTITUTION: When a head arm 7 is to be mounted on a base plate 1, a rotary shaft of the head arm 7 is fixed to a recessed part 8 integrally formed with the base plate 1 while a front end of the head arm 7 is projected outward from a notch part 26. Then, the head arm 7 is rotated to position heads 18, 18 above magnetic discs 10, 10 and, all the structural parts including a magnetic yoke 20, a flexible printed board 23, etc. are secured to the base plate l. A cover 31 is fitted to the base plate 1. Since the recessed part 8 is formed in a bottom plate 2 of the base plate 12 and the notch part communicating with the recessed part 8 is provided in a side wall 3 of the base plate 1, the head arm 7 is relatively easily attached/detached to the base plate 1.