A machine tool having a tool spindle rotatable about a horizontal axis provided with first and second tool magazines and first and second tool transfer devices. The first tool magazine removably stores a plurality of small diameter tools, with the axes of the small diameter tools extending horizontally. The first tool transfer device transfers the small diameter tools between the first tool magazine and the tool spindle. The second tool magazine is installed on the floor at one side of a column of the machine tool in a horizontal direction perpendicular to the tool spindle and removably stores a plurality of large diameter tools, with the axes of the large diameter tools extending vertically. The second tool magazine is provided with a turnover mechanism, which pivotally moves a large diameter tool indexed to a large tool removal position so as to lay the large diameter tool horizontally in parallel relation with the tool spindle. The second transfer device is movable between the large tool removal position and the tool spindle so that a new large tool is removed from the second tool magazine at the large tool removal position to be inserted into the tool spindle, while a used large tool is taken out from the tool spindle to be returned to the second tool magazine at the large tool removal position.
