A synchronizer apparatus for a transmission includes first and second shifting gears rotatable on output shaft, a hub spline-coupled with output shaft and having a slope portion having slanted surfaces, a sleeve spline-coupled with hub, a key coupled with the slope portion, first and second clutch gears disposed between hub and shifting gears, first and second outer rings and first and second inner rings disposed between hub and clutch gears, first and second synchronizer cones disposed between first inner and outer rings and second inner and outer rings and engaged with clutch gears, a poppet ball unit disposed at sleeve and contacting key through a poppet ball, first and second push blocks disposed at slanted surfaces, and first and second push springs inserted in spring holes formed at push blocks. In addition, the push springs is abutted by key such to force push blocks to tightly contact slanted surfaces.