1. A lumber inspection method executed by a programmed processor, comprising: obtaining a color image of lumber captured by imaging means; obtaining a color distribution of the color image captured by the imaging means; comparing the obtained color distribution with a predetermined color distribution of normal lumber; judging the obtained color distribution as an abnormal one when it is deviated from the color distribution of normal lumber by a predetermined value or more; and detecting an area on the surface of the lumber captured by the imaging means whose abnormal color distribution has a value greater than the predetermined value as a defect of the lumber.