A ball joint, comprising: a ball stud that has a shaft and a spherical head provided at one end of the shaft; a tubular housing that has an opening, and that holds therein the spherical head with the shaft projected from the opening; an elastic tubular boot that is fitted at one end onto an end portion of the housing and fitted at the other end onto a middle portion of the shaft, thereby covering the opening; and a fixing ring that is fitted onto the one end of the boot to fix the one end of the boot to the end portion of the housing, wherein an annular groove, in which the one end of the boot is fitted, is formed in an outer periphery of the end portion of the housing so as to extend in a circumferential direction of the housing, the housing has an annular projection that extends in the circumferential direction of the housing, the annular groove has a cylindrical bottom face, and an annular first side face that connects a first end of the bottom face in an axial direction of the housing to an outer periphery of the housing, the annular first side face is formed as a sloping face that slopes upward in a direction toward the radially outer side, the one end of the boot is formed in an annular shape, and has a first portion that faces the bottom face and a second portion that faces the first side face, and when the fixing ring is fitted onto the first portion, the first portion is pushed against the bottom face and the second portion is pushed against the first side face, and the second portion has an annular protrusion that is engaged with the first side face and elastically contracted when the one end of the boot is fitted in the annular groove, and an upper surface of the annular protrusion is formed as a sloping face that slopes upward in a direction toward the radially outer side, and an inclination angle of the upper surface of the annular protrusion with respect to a lateral direction is greater than an inclination angle of the first side face with respect to a lateral direction, and a radial width of the annular protrusion is smaller than a projection amount, by which the annular projection projects from the cylindrical face of the annular groove, characterized in that the annular protrusion is a single annular protrusion having a triangular sectional shape, one side of which extends in the radial direction of the housing, so as to form a sloping face that slopes towards said first side face in the direction toward the radially outer side and is configured to come in tight contact with said first side face. a ball stud that has a shaft and a spherical head provided at one end of the shaft; a tubular housing that has an opening, and that holds therein the spherical head with the shaft projected from the opening; an elastic tubular boot that is fitted at one end onto an end portion of the housing and fitted at the other end onto a middle portion of the shaft, thereby covering the opening; and a fixing ring that is fitted onto the one end of the boot to fix the one end of the boot to the end portion of the housing, wherein an annular groove, in which the one end of the boot is fitted, is formed in an outer periphery of the end portion of the housing so as to extend in a circumferential direction of the housing, the housing has an annular projection that extends in the circumferential direction of the housing, the annular groove has a cylindrical bottom face, and an annular first side face that connects a first end of the bottom face in an axial direction of the housing to an outer periphery of the housing, the annular first side face is formed as a sloping face that slopes upward in a direction toward the radially outer side, the one end of the boot is formed in an annular shape, and has a first portion that faces the bottom face and a second portion that faces the first side face, and when the fixing ring is fitted onto the first portion, the first portion is pushed against the bottom face and the second portion is pushed against the first side face, and the second portion has an annular protrusion that is engaged with the first side face and elastically contracted when the one end of the boot is fitted in the annular groove, and an upper surface of the annular protrusion is formed as a sloping face that slopes upward in a direction toward the radially outer side, and an inclination angle of the upper surface of the annular protrusion with respect to a lateral direction is greater than an inclination angle of the first side face with respect to a lateral direction, and a radial width of the annular protrusion is smaller than a projection amount, by which the annular projection projects from the cylindrical face of the annular groove, characterized in that the annular protrusion is a single annular protrusion having a triangular sectional shape, one side of which extends in the radial direction of the housing, so as to form a sloping face that slopes towards said first side face in the direction toward the radially outer side and is configured to come in tight contact with said first side face.