  The magnetization curves as a response of sweeping magnetic field in the
thermal environment are investigated using the quantum master equation. In a
slow velocity region where the system almost behaves adiabatically, the
magnetic plateau appears which has been observed in the recent experiment of
V$_{15}$ [ Phys. Rev. Lett. (2000)]. We investigate its mechanism and propose
that this phenomenon is quite universal in the quasi-adiabatic transition with
small inflow of the heat, and we call it 'Magnetic Foehn Effect'. We observe
the crossover between this mechanism and the Landau-Zener-St\"{u}ckelberg
mechanism changing the velocity. Some experiment is proposed to clarify the
inherent mechanism of this effect.
