A disc drive apparatus comprises: a spindle motor 20 for hold a disc 2 and for rotate it; an optical head 7 for conducting either one of recording or reproducing of information onto/from the disc; a driver unit for moving the optical head in a radial direction of the disc; and a unit mechanism chassis 8 for supporting those thereon, fixedly, wherein the spindle motor is constructed with a rotational drive means made of a magnetic circuit, a rotor portion and a rotation shaft, which define a rotation body, and a spindle motor attachment plate to hold those thereon, and the spindle motor attachment plate has a semi-circular shaped plate configuration in a part thereof, nearly fitting to an outer diameter the rotor portion, in a direction where the optical head is provided, and has a step-like structure in thickness direction of the plate thereof, within a semi-circular shaped region with respect to a line connecting between corner portions on both ends of that semi-circular shape, and also bridging over that line.