PURPOSE: To provide a thresher having a threshing drum, simply detachable and capable of readily carrying out the maintenance.

CONSTITUTION: This structure for installing and removing a threshing drum 6 of a thresher 2 from the lateral side thereof is constituted by securing threshing drum supporting members 15 and 15 for inserting a driving shaft 14 in the front and rear surface parts of a threshing drum 6, engaging pulley supporting members 8 and 8 with bearings 18 and 19 respectively secured to an inlet side plate 16 arranged in front of the threshing drum 6 and an outlet side plate 17 arranged at the rear thereof, screwing an input pulley 9 for inserting the driving shaft 14 thereinto into the pulley supporting member 8 of the inlet side plate 16, screwing a straw discharge pulley 23 for inserting the driving shaft 14 thereinto onto the pulley supporting member 8 of the outlet side plate 17, extending boss parts (9a) and (23a) of the input pulley 9 and straw discharge pulley 23 to the side of the threshing drum supporting members 15 and 15 and enabling the fitting thereof to the threshing drum supporting members 15 and 15 in the driving rotational direction and disconnection thereof in the reverse rotational direction.