de Haas-van Alphen Oscillation in the Superconducting Mixed State of URu2Si2

We have measured the magnetoresistance and de Haas-van Alphen (dHvA) oscillation of URu 2 Si 2 . From the results of the magnetoresistance measurement, URu 2 Si 2 is concluded to be a compensated metal with closed Fermi surfaces. The dHvA branch, which corresponds to a nearly spherical Fermi surface, has been detected in both the normal and superconducting mixed states. The Dingle temperature or the electron scattering rate is found to be strongly enhanced and the cyclotron effective mass to be correspondingly reduced in the mixed state.