A trimmer comprises a trim knife for trimming the bound material, a blade receiving portion 19 for receiving a blade edge of the trim knife, and an adjustment mechanism 60 configured to adjust a position of the blade receiving portion 19 in such a manner that each of one side 191 and other side 192 of the blade receiving portion 19 moves by not only a same amount but also a different amount and in directions toward and away from the blade edge of the trim knife. Each of positions of the one side 191 and the other side 192 of the blade receiving portion 19 can be adjusted.