A quartz crystal resonator unit has an overall length less than 2.1 mm and a base portion having a length less than 0.5 mm and a width less than 0.55 mm, vibrational arms, and mounting arms connected to the base portion through connecting portions. Each vibrational arm has a first vibrational portion including a first width and a first length within a range of 0.32 mm to 0.72 mm and a second vibrational portion including a second width greater than the first width and a second length less than the first length. A groove is formed in at least one main surface of the first vibrational portions of the vibrational arms, a width of the groove being less than 0.07 mm and a distance in the width direction of the groove being less than 0.015 mm. A width of the mounting arms is less than 0.45 mm and a width of the connecting portion is less than 0.41 mm.