A tablet splitting apparatus (20, 20A, 20B) comprising: a guide having a guide passage (32, 32A) for guiding a falling tablet; a catcher for receiving a tablet moving downward through the guide passage in the middle of the guide passage; a splitting mechanism (40, 40A, 40B) for splitting the tablet received by the catcher; a groove-depth defining member (37, 37A) disposed upstream of the catcher in a tablet moving direction, wherein the guide passage is surrounded by inner surface which includes at least a flat portion upstream of the catcher in the tablet moving direction, and the groove-depth defining member comprises an opposite surface facing the flat portion;, characterized in that it further comprises a thickness adjusting mechanism for varying a relative distance of a facing section between the opposite surface of the groove-depth defining member and the flat portion of the guide passage; a sensor for detecting the tablet passing through a leading portion of the facing section; and a controller (23, 23A, 23B) for controlling the operation of the thickness adjusting mechanism, wherein the relative distance is controllable at least between a narrow state preventing the tablet from passing the facing section and a wide state allowing the tablet to pass the facing section, and the controller performs a thickness measuring operation which activates the thickness adjusting mechanism to expand the relative distance in the narrow state in the presence of the tablet residing upstream of the facing section, and a thickness adjusting operation which halts the thickness adjusting mechanism to fix the relative distance based on the detection of the tablet passing through the leading portion of the facing section. a guide having a guide passage (32, 32A) for guiding a falling tablet; a catcher for receiving a tablet moving downward through the guide passage in the middle of the guide passage; a splitting mechanism (40, 40A, 40B) for splitting the tablet received by the catcher; a groove-depth defining member (37, 37A) disposed upstream of the catcher in a tablet moving direction, wherein the guide passage is surrounded by inner surface which includes at least a flat portion upstream of the catcher in the tablet moving direction, and the groove-depth defining member comprises an opposite surface facing the flat portion;, characterized in that it further comprises a thickness adjusting mechanism for varying a relative distance of a facing section between the opposite surface of the groove-depth defining member and the flat portion of the guide passage; a sensor for detecting the tablet passing through a leading portion of the facing section; and a controller (23, 23A, 23B) for controlling the operation of the thickness adjusting mechanism, wherein the relative distance is controllable at least between a narrow state preventing the tablet from passing the facing section and a wide state allowing the tablet to pass the facing section, and the controller performs a thickness measuring operation which activates the thickness adjusting mechanism to expand the relative distance in the narrow state in the presence of the tablet residing upstream of the facing section, and a thickness adjusting operation which halts the thickness adjusting mechanism to fix the relative distance based on the detection of the tablet passing through the leading portion of the facing section.