A synchronizer apparatus for a transmission includes: first and second shifting gears rotatable on an output shaft; a hub spline-coupled with the output shaft and having a slope portion having slanted surfaces; a sleeve spline-coupled with the hub; a key coupled with a recess of a slope portion; first and second clutch gears disposed between the hub and shifting gears; first and second outer rings and first and second inner rings disposed between the hub and clutch gears; first and second synchronizer cones disposed between the first inner and outer rings and the second inner and outer rings; a poppet ball unit disposed at the sleeve and contacting the key through a poppet ball; first and second push blocks disposed at the slanted surfaces; and first and second push springs inserted in spring holes formed at the push blocks to force push blocks to the slanted surfaces.