A test has been made on the shock pressure of breaking waves against a vertical wall on a steep slope, investigating the time lag between the instant of peak pressure and that of water contact with a pressure transducer mounted on a vertical wall. The test revealed the existence of air in front of the pressure transducer at the instanto of shock pressure for most cases. This supports the Bagnold's air compression theory for the shock pressure. With a pressure transducer of semiconductor strain gage type with the matural frequency of 6600 c/s, peak pressures of 60 to 1340 gr/c㎡ were recorded on an oscilloscope for waves of some 10 cm in height. The duration time of shock pressure ranging from 0.0012 to 0.02 seconds was inversely proportional to its peak value; the time integral of a shock pressure was confirmed to come within a certain range.