A transmission synchronizer apparatus may include a sleeve coupled to an external peripheral surface of a hub gear while moving axially, and clutch gears disposed at both sides of the hub gear, each selectively synchronizing and engaging with the sleeve according to the axial movement of the sleeve, wherein the transmission synchronizer includes a plurality of main sleeve teeth protruding radially from an internal peripheral surface of the sleeve while being distanced from one another, and an auxiliary sleeve tooth protruding radially from the internal peripheral surface of the sleeve while protruding from one side of each of the main sleeve teeth to have a height lower than the main sleeve tooth.