To provide an AT-cut quartz-crystal vibrating piece in which size of the vibrating unit is adjusted to appropriate value, so that the unnecessary vibration is inhibited and degradation of its characteristics is prevented. A mesa-type AT-cut quartz-crystal vibrating piece for vibrating at 38.400 MHz comprising a rectangular excitation unit (31) and a peripheral region (32) formed on the periphery of the excitation unit and having a thickness less than the thickness of the excitation unit, is represented by the following equation: Mx/Gx=0.09×n−0.06 (n: natural number) . . . (1). Length of a crystallographic x-axis direction of the excitation unit is defined as Mx (mm) and length of the crystallographic x-axis direction of the peripheral region and excitation unit is defined as Gx (mm).