According to one embodiment, a laser annealing method includes: detecting an intensity distribution of a laser light formed as a line beam by a line beam optical system; dividing width in short axis direction of the line beam in the detected intensity distribution by number of times of the irradiation per one site and partitioning the width; and calculating increment of crystal grain size of a non-crystalline thin film for energy density corresponding to wave height of the partitioned intensity distribution, and summing the increments by number of times of pulse irradiation, when energy density of the laser light is larger than a threshold, the crystal grain size of the non-crystalline thin film taking a downward turn at the threshold, the increment summed before the energy density exceeds the threshold being set to zero.