A complex synchronizer may include a first connection gear and a second connection gear that are fixed to a shaft; a first external gear having a first sleeve, which can engage with the first connection gear by sliding along the shaft, and rotatably mounted on the shaft; a second external gear having a second sleeve, which can engage with the second connection gear by sliding along the shaft, and rotatably mounted on the shaft; and an intermediate gear being able to connect or disconnect the first sleeve and the second sleeve, depending on axial sliding states of the first sleeve and the second sleeve.