The invention provides a measuring method of a tread wear amount by using a pneumatic tire in which a mark portion is formed on a part of a tread surface, the mark portion changing its surface shape as wear progresses. Image data of the mark portion is detected, the detected data is inputted in processor means, and then the tread wear amount is determined by use of either ratio referential data of the mark portion inputted in advance and corresponding to the wear amounts of the tread surface, or a formula inputted in advance for computing the wear amount, thereby facilitating automatic measurement of the tread wear amount.