High-frequency electromagnetic response in the mixed state of YBa 2Cu 3O y was investigated by measuring the complex surface impedance Z s = R s + iX s in an intense magnetic field H//c. The field- and frequency-dependence of R s and X s was in good agreement with the Coffey-Clem model. From the data, we estimated that the vortex viscosity at 10 K is about 5 × 10 −7 Ns/m 2. This value indicates that the vortex core of YBa 2Cu 3O y is not in the superclean limit. In addition, we observed two kinds of anomaly in the field dependence of Z s at the vortex melting transition, and another field corresponding to the rapid increase of magnetization reported by Nishizaki, respectively.