The present invention resides, in a magnetic disk certifier for detecting one of a spike error and a positive modulation error on the basis of a read signal obtained by reading out by an MR head a test data written on a track of a magnetic disk as a first error of said magnetic disk and detecting one of a missing error and a negative modulation error as a second error, a method and apparatus for detecting a thermal asperity error of said MR head, by of detecting the first error and the second error in positions on said track, the positions having a recording length of a plurality of bits of the test data and judging a thermal asperity error when the first error and the second error are detected in one and the same position of the track.