Synthetic resin bottle, and a combination of regularly used container and refill container

A blow molded synthetic resin bottle that allows for volume-reducing deformation, the blow molded synthetic resin bottle comprising:

• a neck;
• a cylindrical body configured to have a thin peripheral wall, including three ridge lines formed in a mountain fold at approximately equal intervals in a vertical direction, the thin peripheral wall includes three panel walls connected to one another, with each panel wall being bordered by two adjacent ridge lines, each panel wall having a valley fold line diagonally connecting an upper end of one ridge line to a lower end of another adjacent ridge line such that three valley fold lines are disposed obliquely in parallel along a circumferential direction, and the volume-reducing deformation of the body occurs in response to depressurization inside the bottle;
• a tapered cylindrical shoulder having a lower end portion deforming into a first triangular shape with the apexes of the first triangular shape being formed by upper ends of the three ridge lines in response to an occurrence of the volume-reducing deformation;
• and
• a bottom configured to deform into a second triangular shape with the apexes of the second triangular shape being formed by lower ends of the three ridge lines in response to an occurrence of the volume-reducing deformation.