A vertical machining center comprising: a base; a column fixed to the base and vertically standing on the base, the column having a recessed part; a splash cover fixed to the base, the splash cover defining a machining chamber to enclose a machining zone and to separate from a non-machining zone, the splash cover including: a back side cover disposed on the back side of the machining zone; an upper side cover disposed on the upper side of the machining zone, the upper side cover being joined to the back side cover; and a peripheral cover surrounding a periphery of the machining zone, the peripheral cover being joined to the upper side cover; a Z-axis guiding rail disposed on the column and being vertically directed toward the base, the Z-axis guiding rail being partially exposed to the machining zone; a Z-axis driving device disposed in the column, the Z-axis driving device being partially exposed to the machining zone, the Z-axis driving device being housed in the recessed part; a spindle head to be guided by the Z-axis guiding rail and to be driven vertically by the Z-axis driving device; a spindle head opening formed in the splash cover so that the spindle head can pass through the spindle head opening, the spindle head opening being configured so that the spindle head can pass through the upper side cover, and the Z-axis guiding rail and the Z-axis driving device are exposed to the machining zone through the back side cover; and an extendable cover provided between an edge of the spindle head opening and the spindle head, the extendable cover being configured to cover parts of the Z-axis guiding rail and the Z-axis driving device, the parts being exposed to the machining zone; characterised in that a part of the back side cover is formed by the column; the back side cover having a right back cover and a left back cover, the right back cover being joined to a right side surface of the column and joined to a right side rear part of the peripheral cover; and the left back cover being joined to a left side surface of the column and joined to a left side rear part of the peripheral cover. a base; a column fixed to the base and vertically standing on the base, the column having a recessed part; a splash cover fixed to the base, the splash cover defining a machining chamber to enclose a machining zone and to separate from a non-machining zone, the splash cover including: a back side cover disposed on the back side of the machining zone; an upper side cover disposed on the upper side of the machining zone, the upper side cover being joined to the back side cover; and a peripheral cover surrounding a periphery of the machining zone, the peripheral cover being joined to the upper side cover; a back side cover disposed on the back side of the machining zone; an upper side cover disposed on the upper side of the machining zone, the upper side cover being joined to the back side cover; and a peripheral cover surrounding a periphery of the machining zone, the peripheral cover being joined to the upper side cover; a Z-axis guiding rail disposed on the column and being vertically directed toward the base, the Z-axis guiding rail being partially exposed to the machining zone; a Z-axis driving device disposed in the column, the Z-axis driving device being partially exposed to the machining zone, the Z-axis driving device being housed in the recessed part; a spindle head to be guided by the Z-axis guiding rail and to be driven vertically by the Z-axis driving device; a spindle head opening formed in the splash cover so that the spindle head can pass through the spindle head opening, the spindle head opening being configured so that the spindle head can pass through the upper side cover, and the Z-axis guiding rail and the Z-axis driving device are exposed to the machining zone through the back side cover; and an extendable cover provided between an edge of the spindle head opening and the spindle head, the extendable cover being configured to cover parts of the Z-axis guiding rail and the Z-axis driving device, the parts being exposed to the machining zone; characterised in that a part of the back side cover is formed by the column; the back side cover having a right back cover and a left back cover, the right back cover being joined to a right side surface of the column and joined to a right side rear part of the peripheral cover; and the left back cover being joined to a left side surface of the column and joined to a left side rear part of the peripheral cover.