Poloidal shear flow, driven by the radial electric field, decreases the beta limit determined by the ideal interchange mode in a heliotron/torsatron configuration with unfavourable curvature. The reduction factor is estimated as (1-(kθvE')2/(k||'vpA)2) for localized modes, where kθ, is the wave number in the poloidal direction, k|| is the parallel wave number along the magnetic field, vE and vpA are the poloidal shear flow and the poloidal Alfven velocities, respectively, and primes denote the radial derivative. For Δ > LE, the growth rate of the m=1 mode becomes smaller than that for vE = 0; however, the stabilizing effect disappears and the poloidal shear flow enhances the growth rate for Δ < LE (Δ is the radial mode width for vE=0 and LE is the scale length of shear flow)