When waviness having a wavelength component of 10 to 500 μm in the circumferential direction of a main surface of a disk-shaped substrate is acquired and slopes are acquired from the waviness at an interval of 50 to 100 μm, the substrate being used in a magnetic disk on which recording or reading is performed using a DFH head, an average value of absolute values of the slopes is 0.45×10−4 or less. This magnetic-disk substrate is used in a magnetic disk and a magnetic-disk drive device.