A machine tool has a support table provided on an upper surface of a bed to support a work, a Z-axis saddle, a machining unit mounted on the Z-axis saddle and provided with a machining head, a compartment cover arranged on the bed to compartment a region and a machine region, an opening window provided on the compartment cover to permit the machining head to go in and out therethrough, and a seal member in the form of a closed ring to be mounted on an inner peripheral edge of the opening window of the compartment cover, wherein an entire periphery of an outer peripheral surface of the machining head is brought into sliding contact with an inner peripheral edge of the seal member in a state, in which the machining head is moved into the machining region from a tool exchange position within the machine region.