A positioning device capable of increasing the amount of diametrical expansion of an engagement member to be engaged in a cylindrical hole in a workpiece, and capable of performing positioning while not exerting any force on the workpiece in the downward direction positions a cylindrical hole (Ha) in a workpiece (W) in a first direction while permitting shifting thereof in a second direction, the device comprising: the insertion shaft member; a hydraulic cylinder for driving the insertion shaft member; a pair of engagement members; a taper engagement mechanism for shifting the engagement members to increase their diameter; and a movement permitting holding mechanism that, along with supporting the engagement members, so that they cannot shift in the first direction but are capable of shifting in the second direction.