An optical disc drive 1 includes a spindle motor 11 for rotating the optical disc 2 and a spindle driver for driving the spindle motor 11 equipped with a rotation number counter 232 for measuring the rotation number of the spindle motor 11. The first servo bursts are at least substantially radially aligned with each other, and have low frequency regions therebetween. The base plate is, in turn secured to the mounting post and has upper and lower flanges which are engaged by first and second retainers on each of the base brackets. And within a groove passing through insulating layers sandwiching the metal wiring layer from above and below the same as well as on one of the insulating layers there is formed a capacitive element C. Each sensor array has a plurality of ultrasonic sensors with each ultrasonic sensor positioned to direct a respective ultrasonic sensory pulse in a respective direction.