Abstract: The present invention relates to an automatic tool assembly apparatus for a modular constituted tool. The automatic tool assembly apparatus has a tool stock unit for storing a plurality of types of modular constituted tools for a machine tool disassembled in units of modules, a tool transfer unit for assembling a predetermined modular constituted tool from a plurality of modules in the tool stock unit and conveying the assembled modular constituted tool from the tool stock unit, and a control unit. The tool stock unit holds the modular constituted tool disassembled in units of modules. The stock position of each module required for assembling a tool requested by the machine tool is indexed from a signal from the control unit. The modules are then sequentially positioned at the assembly position. The tool transfer unit has an assembly head which is movable in a direction to approach and separate from the assembly position in response to an instruction from the control unit.