A main spindle apparatus has a main spindle having a through hole, a main spindle head rotatably supporting the main spindle therein, a tool holder clamp unit including an unit main body having a tool mounting hole to which a tool holder is mounted, a clamp mechanism for clamping the tool holder and a bias mechanism for biasing the clamp mechanism in a clamping state and a clamp release mechanism provided on the main spindle head side and releasing the clamping state of the clamp mechanism by operating the bias mechanism, wherein the tool holder clamp unit is mounted on the though hole of the main spindle so that the tool holder clamp unit is detachable from front side of the main spindle.