PURPOSE: To provide an expandable grain tank apparatus capable of expanding the grain tank of a combine harvester to increase the volume of the tank and correcting the weight balance of a traveling machine body disturbed by the increase in the volume of stored grains to enable stable traveling motion.

CONSTITUTION: This expandable grain tank apparatus of a combine harvester has the following construction. A grain tank 2 and a thresher 3 are mounted on a mobile vehicle 1 at the right and left sides along the traveling direction. The thresher 3 is attached to a shifting apparatus 4 to enable the lateral motion of the thresher 3 to approach or separate relatively to the grain tank 2. The grain tank 2 is composed of a tank main body 2a containing a grain discharging and transferring apparatus 5 and fixed to the mobile vehicle 1 and a tank adjusting part 2b shiftable in lateral direction relative to the tank main body 2a to change the inner volume of the tank. The tank adjusting part 2b is connected to the thresher 3 and movable in lateral direction.