Abstract: The device for piercing thread staples into folded sheets travelling in a plane includes an endless chain travelling at the same speed above the sheet feeding plane and supporting at regular intervals thread feeding plates which in turn support for a reciprocating movement a needle carrier with two needles having bifurcated points for engaging a thread length in the plate. The needle carrier supports two cam following rollers cooperating, respectively, with a stationary cam assembly above the chain. The cam assembly includes a lower cam for imparting a lifting movement to the needle carrier and a pivotable upper cam for imparting a lowering movement to the carrier against the sheet to be stitched. The upper cam cooperates with a solenoid which urges the same into its working position, and with a tensioning spring which urges the cam into its inoperative position against the force of the solenoid.