Disclosed is a tube expander in which a mandrel itself is in the form of a short metal bar and the mandrel is fitted into a cage having loosely fitted therein a plurality of rollers adapted to perform a tube expanding function. The forward end of the mandrel includes a threaded portion formed in the outer surface thereof and engageable with a threaded guide disposed inside the forward end portion of the cage and adapted to be rotatable with the cage and slidable axially thereof, and the rear end of the mandrel is engaged with a mandrel drive which axially slidably rotates the mandrel. A suitable number of shanks may be added to the mandrel drive as occasions demand, whereby the desired tube expanding operation can be performed without the need to apply any propelling force to the mandrel drive but simply by externally applying a turning force to the mandrel drive or the shanks.