The invention provides a variable valve train for an internal combustion engine, in which attachment and detachment of a rotation drive source can be performed without an affect on a transmission mechanism and environment. The variable train system comprises a variable valve system that is fixed to a cylinder head and implements variable control on valve drive outputs according to displacement that is inputted to a control input member; a rotation drive source that outputs control rotation for setting valve properties from an output shaft; and a transmission mechanism that is located on the side of the variable valve system, receives the control rotation outputted from the output shaft with an input shaft, and transmits the control rotation to the control input member, wherein the rotation drive source is detachably fixed to an engine body; the output shaft of the rotation drive source is coupled to the input shaft by using a coupling that moves the output shaft toward the input shaft and disengageably couples the output shaft to the input shaft; and the coupling transmits the rotation of the output shaft to the input shaft while allowing misalignment between the output shaft and the input shaft.