[en] The PFring of the synchrotron radiation experimental facility in National Laboratory for High Energy Physics is the 2.5 GeV electron 1 positron storage ring for the experiment on the utilization of synchrotron radiation. In single bunch operation, since the current per bunch becomes large, the high frequency loss of bunch beam due to the unevenness of beam vacuum duct wall cannnot be ignored. The duct elements having cavity structure can become the cause of bunch combination type instability. Also in the multi-bunch operation of the PFring, the slight deterioration of beam life became to be observed. In order to evaluate the impedance of the vacuum duct elements of the PFring, the measurement and the numerical calculation were begun. The measurement of impedance was carried out by wire method. The results of measurement and the evaluation of the impedance of the PFring are reported. Resonance and beam instability are discussed. In the PFring, based on the past operation experience and the results of this research, the countermeasures to the unevenness of beam path wall were advanced, and as for the shield of the gate valve and the flange gap, problems were eliminated. (K.I.)