An optical disk recording/reproduction device and a disk determination method for an optical disk recording/reproduction device. The disk determination method compares with a threshold value a drive value when the arrival of a focus point at a recording surface is detected during a focus search relative to an optical disk (301) or a drive value during an on-focus state. By this, two or more kinds of optical disks having different substrate thicknesses are determined. The threshold value is determined based on a drive value during an on-focus state when one or more kinds of disks out of the two or more kinds of optical disks become reproducible.