A transmission for a vehicle may include an input shaft connected to a power source, an output shaft disposed in parallel with the input shaft, a synchromesh type shift mechanism including at least two pairs of external gear pairs and a synchronous device which may be installed on the input shaft and the output shaft, and a clutch device provided on a power transfer path connected from the input shaft to the output shaft through the external gear pair and configured to connect the synchronous device in parallel with the power transfer path.