Constant pressure cooker and fastener

Disclosed herein is a constant pressure cooker which has a coupling steel ring attached to the peripheries of the cover of the cooker and the cooker body and there is an opening at the coupling ring so that an adjustment of the opening can adjust the tightness of the cooker in its application. The said coupling steel ring has a springband on its upper ring surface or the steel ring itself is made of elastic steel material so that whenever the pressure cooker is tightly sealed and the steam therein has reached its predetermined saturated pressure, there is a upper thrust force, then, the spring band at the coupling steel ring moves upwards or the elastic steel ring expands so that the saturated steam in the cooker is released and the steam pressure in the cooker keeps constant.