Transmission of working vehicle

A transmission of a working vehicle, the transmission comprising:

• an ultra-low speed gearbox having an ultra-low speed shift rod which moves axially and an ultra-low speed shift fork which is coupled to the ultra-low speed shift rod, wherein the ultra-low speed gearbox performs a ultra-low speed shifting by receiving power from a main gearbox which performs a first speed shifting;
• a sub-gearbox which is driven by the power transmitted from the ultra-low speed gearbox, the sub-gearbox having a first sub-shift rod and a second sub-shift rod which move axially and are parallel to each other, and a first sub-shift fork and a second sub-shift fork which are coupled respectively to the first sub-shift rod and the second sub-shift rod;
• and
• a stopper which is disposed between the ultra-low speed gearbox and the sub-gearbox, the stopper being configured to prevent an ultra-low speed shift stage engagement of the ultra-low speed gearbox by blocking an axial movement of the ultra-low speed shift rod at an end of the ultra-low speed shift rod when the first sub-shift rod moves to engage in a predetermined sub-shift stage of sub-shift stages.