A twin clutch transmission for reducing the frequency of hit noise occurring in a dog clutch when shifting from neutral to a first speed. The twin clutch transmission includes a plurality of pairs of gears according to the number of speed stages between dual-structured main shafts and a countershaft. A twin clutch is disposed on the main shafts wherein a rotational drive force of an engine is transferred or blocked. A transmission between adjacent speed change gears on each shaft by a dog clutch occurs with a shift drum for switching between an engagement condition of the dog clutch by driving a shift fork. The patterns of guide grooves in the shift drum are formed wherein the dog clutches for a first speed gear and a second speed gear engage simultaneously when the shift drum is rotated by predetermined angles to shift from neutral to the first speed.