To provide a tire width measuring device capable of reducing the labor of tire width measuring work, preventing the deterioration of measurement precision, and preventing the wrong reading of a measurement value by a worker and the reading error.

Locks 54A and 54B are slid on a rail guide 44 supported by a body frame, and each lock 54A and 54B is brought into contact with the side part of an OR tire 14. Since moving tables 48A and 48B are in contact with the rail guide 44 through a ball or roller, the frictional forces of the both are reduced to prevent the wear of the rail guide 44. The space between the locks 54A and 54B is measured by a digital tape measure 56 to measure the tire width.