A transmission comprising a gear module and a shift fork module with the shift fork module to be mounted on the gear module to complete the transmission in which the gear module comprises a 1 st shaft to which a driving power of a driving source of a vehicle is inputted and having a plurality of forward drive gears (G1 to G5) of different diameters corresponding to gear ratios and a reverse drive gear; a 2 nd shaft having a plurality of forward driven gears (G1' to G5') and a reverse driven gear (G6') respectively mating with the forward drive gears (G1 to G5) and the reverse drive gear; a plurality of stator members (S1 to S3) for selectably synchronizing the forward drive gears (G1 to G5) or reverse driven gear (G6') for transmitting the driving power from the 1 st shaft to the 2 nd shaft; and a 3 rd shaft having a reverse idle gear (Ga) which can be mated with the reverse drive gear and reverse driven gear (G6') when performing a gear shift operation to a reverse stage and transmit the reversed driving rotation to the 2 nd shaft; the 1 st shaft, 2 nd shaft and 3 rd shaft being supported by a case member formed by a second case member element assembled to a first case member element, wherein the shift fork module comprises: a plurality of shift fork shafts (T1 to T3) which are provided with shift forks (F1 to F3) engageable with respective stator members (S1 to S3) and can select the forward drive gears (G1 to G5) and forward driven gears (G1' to G5') transmitting the driving power from the 1 st shaft to the 2 nd shaft by moving the stator members (S1 to S3) to any one of the forward drive gears (G1 to G5) and forward driven gears (G1' to G5') when performing a gear shift operation; an operation member mounted on the shift fork shaft and operating when performing a gear shift operation to the reverse stage; a reverse lever (4 or 4') connected to the operation member, one end of the reverse lever (4 or 4') engaging the reverse idle gear (Ga) when assembled to the gear module, and the reverse lever (4 or 4') being able to switch the reverse idle gear (Ga) between a state in which it is mated with the reverse drive gear and the reverse driven gear (G6') and a state in which it is not mated with the reverse drive gear and the reverse driven gear (G6'); a base member for slidably supporting the shift fork shafts (T1 to T3), wherein the reverse lever (4, 4') is rotationally mounted on a mounting plate (8, 8') and the mounting plate (8, 8') is secured on the base member; characterized in that the base member is mounted on the second case member element of the case member, wherein the base member is formed with a guiding groove, wherein the guiding groove is able to engage a projection formed on a mounting surface of the gear module for mounting the base member and structured to guide and position the base member at a predetermined position when the shift fork module is assembled to the gear module, wherein an upper surface of an edge portion of the second case member element is formed so that it is flush with the mounting surface of the base member. a plurality of shift fork shafts (T1 to T3) which are provided with shift forks (F1 to F3) engageable with respective stator members (S1 to S3) and can select the forward drive gears (G1 to G5) and forward driven gears (G1' to G5') transmitting the driving power from the 1 st shaft to the 2 nd shaft by moving the stator members (S1 to S3) to any one of the forward drive gears (G1 to G5) and forward driven gears (G1' to G5') when performing a gear shift operation; an operation member mounted on the shift fork shaft and operating when performing a gear shift operation to the reverse stage; a reverse lever (4 or 4') connected to the operation member, one end of the reverse lever (4 or 4') engaging the reverse idle gear (Ga) when assembled to the gear module, and the reverse lever (4 or 4') being able to switch the reverse idle gear (Ga) between a state in which it is mated with the reverse drive gear and the reverse driven gear (G6') and a state in which it is not mated with the reverse drive gear and the reverse driven gear (G6'); a base member for slidably supporting the shift fork shafts (T1 to T3),