The mechanical loss and frequency shift for tilt-oscillation of superc
onducting kappa-(BEDT-TTF)2Cu(NCS)2 Crystal plate in the magnetic fiel
d were measured as a function of temperature near T(c). For a magnetic
field perpendicular to the two-dimensional plane, the loss increased
below T, and exhibited a maximum, accompanying the resonance frequency
up-shift. The experimental results are interpreted in terms of therma
lly activated fluxoid flow.