We have developed monolithic SOI type 1200V level shifters for inverter driver ICs, which are based on a new design concept of cascaded 120V LDMOSFETs. In order to clarify the problem of blocking voltage lowering against a high dV/dt surge, we have analyzed transient behavior of monolithically cascaded LDMOSFETs by numerical simulations and experiments. As a result, we have established a new design concept including device layouts and circuits, which minimizes breakdown voltage lowering at very high dV/dt of 20kV/μs. This concept is expected to be a key technology enabling 1200V SOI inverter driver ICs for harsh applications such as automotive electronics.