A method for measuring the tread thickness from the radially outermost belt surface within a tire to the tread surface accurately by ultrasonic measurement whatever the type of belt material is. The method includes the steps of measuring a profile of the tread surface, identifying the bottom positions of circumferentially extending grooves from the profile measured, estimating the depths from the groove bottoms to the belt surface from the identified groove bottoms and pre-created tire design data, predicting thicknesses by calculating predicted thicknesses from the tread surface to the belt surface from the relationship between the thicknesses from the identified groove bottoms to the tread surface in the profile and the estimated depths from the groove bottoms to the belt surface, and setting amplification factors for received reflected waves according to a reflection factor-thickness data map prepared beforehand in correspondence to the predicated thicknesses.