The present disclosure provides a mesa-type AT-cut quartz-crystal vibrating piece, in which amount of the vibrating unit is adjusted to appropriate amount, in order to inhibit unnecessary vibration and to prevent degradation. The mesa-type AT-cut quartz-crystal vibrating piece (30) for vibrating piece vibrates at 38.400 MHz comprises a rectangular excitation unit (31), a peripheral region (32) formed in periphery of the excitation unit and thinner than the excitation unit. The thickness difference h (μm) between one principal surface of the excitation unit and the adjacent peripheral region is obtained by the following equation: h=(0.2×Mx)−143 The length of the x-axis direction of the crystallographic X-axis is Mx (μm).