Applying stepwise compressive disturbances of only 1.0 bar from 1.5 to 2.5 bar to a steamwater two-phase medium containing an initial void fraction up to 17%, very strong secondary pressure pulses with peaks that could approach 100 bar lasting a few milliseconds were observed to evolve by the impulse of hammer action, in a shock tube type vertical experimental device constituted of a stainless steel pipe of rigid structure, 40 mm inner diameter and about 2.5 m long. The intensity of these secondary shock pulses agreed fairly well with values based on Karplus' theoretical treatment slightly modified. The measured propagation velocity of the primary disturbances showed good agreement with that based on the completely condensed shock wave model proposed by Karplus. Comparing the present results with those obtained in a preliminary experiment by the present authors, it is concluded that the propagation of the primary shock pulse waves should be distinguished from that of usually considered pressure waves with small amplitude. It is suggested that, from considerations of safety, careful attention should be paid in the design of reactors with a low system pressure-e.g. sodium cooled FBR-to the possibility of such strong secondary pressure pulses being generated by hammer action. Appreciable damage was actually sustained in the experiment by detectors of semiconductor piezo type with 5 mm outer diameter and 10kg/cm 2 rating.