Flush toilet

1. A flush toilet, comprising a bowl including a drainage outlet portion in a bottom portion and a discharge port in an upper inner surface portion, the flush toilet being configured to wash an inner surface of the bowl by allowing washing water discharged from the discharge port to flow down while swirling along the inner surface of the bowl, wherein

the inner surface of the bowl comprises a guide passage configured to control flow of the washing water that flows down along the inner surface of the bowl,

the guide passage comprises a concave surface that is concave in at least one vertical cross section of the bowl,

the guide passage is helical in shape and becomes gradually lower toward the drainage outlet portion from an upper portion of the bowl, and

in the at least one vertical cross section of the bowl, every inclined tangent on the inner surface in the at least one vertical cross section goes downward toward a drainage outlet portion side.

2. The flush toilet according to claim 1 , wherein a convex curved surface that is convex in the at least one vertical cross section is continuous to a lower side of the concave surface, and a tangent at an inflection point between the concave surface and the convex curved surface is inclined downward toward the drainage outlet portion in the at least one vertical cross section of the bowl.

3. The flush toilet according to claim 1 , wherein the every inclined tangent on the inner surface of the bowl goes downward toward the drainage outlet portion side in every vertical cross section of the inner surface of the bowl.

4. The flush toilet according to claim 1 , wherein the guide passage has a helical shape that reaches the drainage outlet portion in a bottom portion of the bowl from the upper portion of the bowl in two or less turns.

5. The flush toilet according to claim 1 ,

wherein the bowl includes a water guide passage surrounded by a peripheral wall in an upper peripheral edge, and the discharge port is configured to discharge the washing water along a circumferential direction of the bowl, and supply the washing water inside the bowl via the water guide passage,

the water guide passage makes a round along the upper peripheral edge of the bowl, and is inclined downward toward an inner peripheral side, and

the guide passage is for guiding washing water that flows down as a swirling flow from the water guide passage to the drainage outlet portion.

6. The flush toilet according to claim 1 ,

wherein a convex curved surface that is convex in the at least one vertical cross section is continuous to a lower side of the concave surface, and

a curvature radius of the convex curved surface, on one side of the bowl, whose inflection point with the concave surface has a small height from the bottom portion of the bowl is larger than a curvature radius of the convex curved surface, on another side of the bowl, whose inflection point with the concave surface has a large height from the bottom portion of the bowl.

7. The flush toilet according to claim 6 , wherein an inner wall surface on a lower side of the convex curved surface on the other side of the bowl having the small curvature radius has a larger inclination than an inner wall surface on a lower side of the convex curved surface on the one side of the bowl having the large curvature radius.

8. The flush toilet according to claim 6 , wherein a range of the convex curved surface on the one side of the bowl having the large curvature radius is a range in an approximately half round in a circumferential direction.

9. The flush toilet according to claim 2 , wherein the guide passage has a helical shape that reaches the drainage outlet portion in a bottom portion of the bowl from the upper portion of the bowl in two or less turns.