Vibration of a crystal unit is actually mechanical vibration. However ,the crystal unit can be expressed by a two-terminal network if its behavior is electrically converted. The series circuit consisting of L1, C1,and R1 is related to elastic vibration ,while the element C0 connected in parallel to the series arm as a capacitance attributable to the dielectric body of a quartz crystal plate .The resistance R1 is a resonance resistance of the crystal unit at the series resonance frequency. (See Fig.1.)