PURPOSE: To obtain an operation mechanism of a compact manual transmission suitable for automatic assembling by arranging an interlock mechanism to control movement of the other shift rods, when one shift rod is shifted on the side part inside a transmission case.

CONSTITUTION: In a transmission having for example five forward shift steps and one backward shift step, of which respective gear trains 110-160 are selectively let in the power transmitting condition by sliding the sleeves 21b-23b of a first-a third synchronizers 21-23, three shift rods 11-13 are arranged on the upper part of a transmission case in parallel with an output shaft 3, and rotatably supported with front and rear holders 14, 24. An interlock mechanism 15 is provided in one body with the front holder 14, and a click stop mechanism is arranged on the lower part of the mechanism 15. By such constitution, an operation mechanism can be reasonably received in the cylindrical transmission case having no upper cover and no intermediate supporting partition wall, to enable automatic assembling.