A synthetic resin cap includes a top wall, a skirt wall, and a tamper evidence band connected to the skirt wall with bridges interposed therebetween. An inner face of the top wall includes an annular inner leg, an annular projection, and an annular outer leg. An intermediate part of the inner leg has a substantially constant transverse thickness in a vertical direction. Inner and outer circumferential faces of the intermediate part have substantially the same curvature of longitudinal section and are arcuate and convex inwardly in a radial direction, and an outer circumferential face of a distal end part of the inner leg has a guide face that guides the container mouth part, and the intermediate part is inclined radially outwardly so that an upper end of the guide face is shifted radially inwardly by 0.1 to 0.6 mm when the inner leg is inserted in the container mouth part.