In active massive star forming regions such as Orion and the Galactic center, the self-gravitational instability of a magnetized gaseous slab plays an important role as a trigger of star formation. In such high external pressure regions, the incompressible mode of self-gravitational instability (Elmegreen & Elmegreen 1978; Lubow & Pringle 1992) becomes dominant. Based on two-dimensional hydrodynamical simulations, Umekawa et al. (1999) proposed “Star formation by merging of the Jeans stable clumps” in a pressure bounded slab. In a magnetized slab confined by external pressure, Nagai et al. (1998) showed by linear analysis that the slab fragments to filaments parallel to the magnetic field lines. Here, we show by nonlinear three-dimensional MHD simulations that the filaments further fragment to Jeans stable clumps.