A ball bearing unit for a turbocharger according to an embodiment includes: an outer ring, a first inner ring, a second inner ring, a first machined retainer and a second machined retainer. The outer ring has an inner peripheral surface provided with an outer ring raceway surface at one end side and an outer ring raceway surface at another end side. At least one of the first machined retainer and the second machined retainer includes a plurality of pockets that hold the plurality of balls in a rollable manner in a circumferential direction. Each of the pockets has a diameter of equal to or more than 1.03 times and equal to or less than 1.07 times of a diameter of the balls.