The flux flow resistivity associated with purely viscous motion of vorticesin high-quality MgB_2 was measured by microwave surface impedance. Flux flowresistivity exhibits unusual field dependence with strong enhancement at lowfield, which is markedly different to conventional s-wave superconductors. Acrossover field which separates two distinct flux flow regimes having differentflux flow resistivity slopes was clearly observed in H//ab-plane. The unusualH-dependence indicates that two very differently sized superconducting gaps inMgB_2 manifest in the vortex dynamics and almost equally contribute to energydissipation. The carrier scattering rate in two different bands is alsodiscussed with the present results, compared to heat capacity and thermalconductivity results.
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