To provide a drying method for terephthalic acid and a horizontal rotary dryer allowing easy performance of mass processing of the terephthalic acid and enabling size reduction by improving drying performance of the dryer. In a method of drying terephthalic acid by using a horizontal rotary dryer, a rotating shell is rotated to make a critical speed ratio α defined by expression 1 and expression 2 become 17 to less than 80% to dry the processing material, Vc=2.21D½  Expression 1 α=V/Vc·100  Expression 2 wherein Vc indicates a critical speed (m/s) of the rotating shell, D indicates an inside diameter (m) of the rotating shell, α indicates the critical speed ratio (%) of the rotating shell, and V indicates a rotation speed (m/s) of the rotating shell.