A machining center in which the moment (M) exerted on a support base (21) and support table (28) during a machinIng operation is remarkably reduced. The machining apparatus includes a pair of guide rails (29,30) extending in the longitudinal direction (X) relative to the workpiece upon which the machining head (32,35) is slidably supported either directly or via a table. The rear one (30) of this pair of guide rails (29,30) is made higher than the other guide rail (29), thereby reducing the reaction forces exerted upon the upper surfaces of the guide rails (29,30), and hence the moment (M) exerted on the supporting base (21).