The history from the recognition of atmospheric pressure to the thermodynamic standard state pressure (SSP) is reviewed. Originally, the SSP was defined to be 101.325 kPa, one atmosphere in terms of a conventional unit, which was based on the normal atmosphere. However, IUPAC recommended 100 kPa as a new SSP in 1981. The normal atmosphere had not just been a standard (prototype) of pressure unit, but a knowledge relating to the recognition of atmospheric pressure and the normal boiling points of chemical substances. This is the reason why the IUPAC recommendation on the SSP has not completely prevailed even in chemistry