The viscous force acting on the magnetic flux in a high-Tc Bi2Sr2Ca2Cu3O10 superconductor was evaluated by applying a pulsed magnetic field. It was found that the magnetic flux intruding into the superconductor moves at a speed of about 50 m/s. The viscous force and the pinning force in a Bi-based high-Tc superconductor are discussed in the framework of the critical state model. The viscosity (η/φ0) of the magnetic flux was estimated to be about 1×105A·s/m3.
