A positioning device that, by engaging an insertion shaft member provided to a base member into a cylindrical hole (Ha) of a workpiece so as to contact said workpiece closely via engagement members, is capable of positioning the workpiece in a first direction that is orthogonal to a vertical axis of the insertion shaft member while permitting shifting of the workpiece in a second direction that is orthogonal to said vertical axis and to said first direction, comprising: a main body member fixed to said base member; the insertion shaft member, that is installed to said main body member and whose upper end portion projects upward from an upper side of the main body member; a fluid pressure cylinder capable of driving said insertion shaft member upward and downward; a pair of engagement members that are disposed on opposite sides of said insertion shaft member in said first direction; a taper engagement mechanism that, by shifting said insertion shaft member downward with respect to said pair of engagement members with said fluid pressure cylinder, causes said pair of engagement members to shift so that their diameter increases; and a movement permitting holding mechanism that, along with supporting said pair of engagement members also holds them so that they cannot shift in said first direction but can shift in said second direction; and further the positioning device being constructed so that, after the upper end portion of said insertion shaft member and the upper portions of the pair of engagement members have been inserted into said cylindrical hole (Ha) of said workpiece via said movement permitting holding mechanism, the external diameter of said pair of engagement members is increased via said taper engagement mechanism by said insertion shaft member being driven downward by said fluid pressure cylinder so that the pair of engagement members come into close contact with said cylindrical hole (Ha) and constructed so that, within a range of tolerance for an internal diameter of said cylindrical hole (Ha), said pair of engagement members are capable of being elastically deformed in a direction to increase their external diameter until said pair of engagement members come into close contact with an inner circumferential surface of said cylindrical hole (Ha); characterized in that said movement permitting holding mechanism supports said pair of engagement members so that they cannot move downward, and in that said taper engagement mechanism comprises a pair of tapered slide surfaces that are formed parallel to said second direction on opposite side portions in said first direction of said insertion shaft member, and a pair of tapered engagement surfaces that are formed on said pair of engagement members so as to be in surface contact with those tapered slide surfaces. a main body member fixed to said base member; the insertion shaft member, that is installed to said main body member and whose upper end portion projects upward from an upper side of the main body member; a fluid pressure cylinder capable of driving said insertion shaft member upward and downward; a pair of engagement members that are disposed on opposite sides of said insertion shaft member in said first direction; a taper engagement mechanism that, by shifting said insertion shaft member downward with respect to said pair of engagement members with said fluid pressure cylinder, causes said pair of engagement members to shift so that their diameter increases; and a movement permitting holding mechanism that, along with supporting said pair of engagement members also holds them so that they cannot shift in said first direction but can shift in said second direction; and further the positioning device being constructed so that, after the upper end portion of said insertion shaft member and the upper portions of the pair of engagement members have been inserted into said cylindrical hole (Ha) of said workpiece via said movement permitting holding mechanism, the external diameter of said pair of engagement members is increased via said taper engagement mechanism by said insertion shaft member being driven downward by said fluid pressure cylinder so that the pair of engagement members come into close contact with said cylindrical hole (Ha) and constructed so that, within a range of tolerance for an internal diameter of said cylindrical hole (Ha), said pair of engagement members are capable of being elastically deformed in a direction to increase their external diameter until said pair of engagement members come into close contact with an inner circumferential surface of said cylindrical hole (Ha); characterized in that said movement permitting holding mechanism supports said pair of engagement members so that they cannot move downward, and in that said taper engagement mechanism comprises a pair of tapered slide surfaces that are formed parallel to said second direction on opposite side portions in said first direction of said insertion shaft member, and a pair of tapered engagement surfaces that are formed on said pair of engagement members so as to be in surface contact with those tapered slide surfaces.