A tablet cassette comprising: a cassette body for storing tablets; and a rotor that is disposed rotatably in a bottom of a tablet container of the cassette body, the rotor having an outer peripheral surface on which a plurality of tablet guide grooves extending in a rotor rotation axial direction are formed, wherein the rotor further comprises: a groove width adjustment mechanism that relatively moves both sides of each of the tablet guide grooves in a circumferential direction of the rotor, wherein the groove width adjustment mechanism comprises: a first movable plate that has a plurality of first adjustment walls, each of the plurality of first adjustment walls having a first side surface that forms one side of each of the plurality of tablet guide grooves; a second movable plate that is disposed rotatably relative to the first movable plate and has a plurality of second adjustment walls, each of the plurality of second adjustment walls having a second side surface that forms the other side of each of the plurality of tablet guide grooves opposed to the first side surface of the first movable plate; and a movable plate moving mechanism for relatively rotating the first and second movable plate. a cassette body for storing tablets; and a rotor that is disposed rotatably in a bottom of a tablet container of the cassette body, the rotor having an outer peripheral surface on which a plurality of tablet guide grooves extending in a rotor rotation axial direction are formed, wherein the rotor further comprises: a groove width adjustment mechanism that relatively moves both sides of each of the tablet guide grooves in a circumferential direction of the rotor, wherein the groove width adjustment mechanism comprises: a first movable plate that has a plurality of first adjustment walls, each of the plurality of first adjustment walls having a first side surface that forms one side of each of the plurality of tablet guide grooves; a second movable plate that is disposed rotatably relative to the first movable plate and has a plurality of second adjustment walls, each of the plurality of second adjustment walls having a second side surface that forms the other side of each of the plurality of tablet guide grooves opposed to the first side surface of the first movable plate; and a movable plate moving mechanism for relatively rotating the first and second movable plate. a groove width adjustment mechanism that relatively moves both sides of each of the tablet guide grooves in a circumferential direction of the rotor, wherein the groove width adjustment mechanism comprises: a first movable plate that has a plurality of first adjustment walls, each of the plurality of first adjustment walls having a first side surface that forms one side of each of the plurality of tablet guide grooves; a second movable plate that is disposed rotatably relative to the first movable plate and has a plurality of second adjustment walls, each of the plurality of second adjustment walls having a second side surface that forms the other side of each of the plurality of tablet guide grooves opposed to the first side surface of the first movable plate; and a movable plate moving mechanism for relatively rotating the first and second movable plate. a first movable plate that has a plurality of first adjustment walls, each of the plurality of first adjustment walls having a first side surface that forms one side of each of the plurality of tablet guide grooves; a second movable plate that is disposed rotatably relative to the first movable plate and has a plurality of second adjustment walls, each of the plurality of second adjustment walls having a second side surface that forms the other side of each of the plurality of tablet guide grooves opposed to the first side surface of the first movable plate; and a movable plate moving mechanism for relatively rotating the first and second movable plate.