  We study theoretically the lattice instability in the spin gap systems under
magnetic field. With the magnetic field larger than a critical value h_{c1},
the spin gap is collapsed and the magnetization arises. We found that the
lattice distortion occurs in the spin-ladder at an incommensurate wavevector
corresponding to the magnetization, while it does not occur in the Haldane
system. At low temperatures the magnetization curve shows a first order phase
transition with this lattice distortion.
