Synthetic resin container having a rectangular cylindrical part and a round cylindrical narrow part

1. A synthetic resin container, comprising:

a mouth part,

a shoulder part,

a trunk part, and

a bottom part,

wherein said trunk part has a round cylindrical narrow part, an upper rectangular cylindrical part present on a side where the shoulder part is present, and a lower rectangular cylindrical part present on a side where the bottom part is present,

the round cylindrical narrow part is formed by narrowing a predetermined height position of said trunk part from a lower end of the upper rectangular cylindrical part and an upper end of the lower rectangular cylindrical part,

an upper end of said round cylindrical narrow part continues to said upper rectangular cylindrical part and a lower end of said round cylindrical narrow part continues to said lower rectangular cylindrical part such that a cross sectional shape thereof becomes closer to a cross sectional shape of said upper rectangular cylindrical part and said lower rectangular cylindrical part respectively while increasing a diameter of said round cylindrical narrow part concentrically from a narrowest portion upwardly and downwardly in a height direction,

a boundary between said upper rectangular cylindrical part and said round cylindrical narrow part is positioned at a closest part to said shoulder part in a corner part of said upper rectangular cylindrical part,

a boundary between said lower rectangular cylindrical part and said round cylindrical narrow part is positioned at a closest part to said bottom part in a corner part of said lower rectangular cylindrical parts, and

the upper rectangular cylindrical part and the lower rectangular cylindrical part have a rectangular shape in a cross section orthogonally crossing the height direction.

2. The synthetic resin container according to claim 1 , wherein the cross section orthogonally crossing the height direction of the narrowest portion of said round cylindrical narrow part has a circular shape.

3. The synthetic resin container according to claim 2 , wherein a ridge line is molded in the boundary between said upper rectangular cylindrical part and said round cylindrical narrow part and in the boundary between said lower rectangular cylindrical part and said round cylindrical narrow part.

4. The synthetic resin container according to claim 2 , wherein a pair of steps which is positioned above and below in the height direction with the narrowest portion of said round cylindrical narrow part being sandwiched therebetween is formed in a circumferential direction.

5. The synthetic resin container according to claim 1 , wherein a ridge line is molded in the boundary between said upper rectangular cylindrical part and said round cylindrical narrow part and in the boundary between said lower rectangular cylindrical part and said round cylindrical narrow part.

6. The synthetic resin container according to claim 5 , wherein the ridge line is a wavy line curved up and down in the height direction.

7. The synthetic resin container according to claim 1 , wherein a pair of steps which is positioned above and below in the height direction with the narrowest portion of said round cylindrical narrow part being sandwiched therebetween is formed in a circumferential direction.

8. The synthetic resin container according to claim 7 , further comprising a rib formed on the round cylindrical narrow part.

9. The synthetic resin container according to claim 8 , wherein the rib is formed at the narrowest portion of said round cylindrical narrow part such that the rib protrudes upwardly and downwardly in the height direction from the narrowest portion of said round cylindrical narrow part.