Quartz crystal blank and quartz crystal resonator unit

A rectangular quartz crystal blank having long sides substantially parallel to a Z′ axis of the quartz crystal blank, and short sides substantially parallel to an X axis of the quartz crystal blank. The quartz crystal blank includes a first center region, a second region and a third region that are adjacent to the first region along a long-side direction, and a fourth region and a fifth region that are adjacent to the first region along a short-side direction. A thickness of the second region and a thickness of the third region are smaller than a thickness of the first region, and/or a thickness of the fourth region and a thickness of the fifth region are smaller than the thickness of the first region, and 19.87≤W/T≤20.36, where W is a length of a short side and T is a thickness.