A synchronizing device of a manual transmission may include a clutch hub splined on a rotation shaft, a sleeve engaged to an external circumferential portion of the clutch hub by interposing a blocking key therebetween, the sleeve being movable axially, a clutch gear having a cone portion and disposed at both sides of the sleeve without an interference with the rotation shaft, a blocker ring frictionally engagable to the cone portion of the clutch gear, and a normal gear and a groove gear formed at an interior circumference of the sleeve, wherein a width of a middle portion of the groove gear may be wider than that of the normal gear, and the middle portion of the groove gear has a key supporting portion and a locking groove formed on the key supporting portion and engaged to an exterior circumference of the blocking key.