A force measuring device adapted to be mounted in a bottom bracket assembly of a bicycle, the bottom bracket assembly including a bottom bracket shell which is connected to a seat tube, a down tube, and chain stays of the bicycle, and which has a right shell segment, a spindle to which a crankset and a chainwheel are connected, which is rotatably mounted relative to the bottom bracket shell about a first spindle axis, and which has a right spindle segment that is spacedly surrounded by the right shell segment, and an antifriction bearing unit which is disposed between the spindle and the bottom bracket shell, said force measuring device comprising: a sleeve shell configured to be inserted between the spindle and the bottom bracket shell, and having a first shell-half outer wall and an inner tubular wall which are radially spaced apart from each other to define a circumferentially extending surrounding clearance, and which are adapted to be in abutting engagement with the right shell segment of the bottom bracket shell and the antifriction bearing unit, respectively; a ring body which is disposed on and movable with said inner tubular wall, and which has an outer ring surface that confronts and is spaced apart from said first shell-half outer wall, said outer ring surface having a strain region which is configured to make a strain displacement corresponding to a treading force exerted on the spindle; a sensor holding unit which defines a sensor activating zone that confronts said strain region, and which is disposed to be unmoved with said first shell-half outer wall; and a hall sensing unit including a sensor disposed in said sensor activating zone and held to be unmoved relative to said sensor holding unit, and a magnetic member disposed to be displaceable with the strain displacement of said strain region, and spaced apart from said sensor by a predetermined distance such that a magnetic field generated thereby is changed when said magnetic member is displaced with the strain displacement so as to give off a signal indicative of the magnitude of the treading force, said force measuring device characterized by a force transmitting mount which is disposed on said strain region, which extends radially toward said sensor activating zone, and which is configured to permit said magnetic member to be mounted therein. a sleeve shell configured to be inserted between the spindle and the bottom bracket shell, and having a first shell-half outer wall and an inner tubular wall which are radially spaced apart from each other to define a circumferentially extending surrounding clearance, and which are adapted to be in abutting engagement with the right shell segment of the bottom bracket shell and the antifriction bearing unit, respectively; a ring body which is disposed on and movable with said inner tubular wall, and which has an outer ring surface that confronts and is spaced apart from said first shell-half outer wall, said outer ring surface having a strain region which is configured to make a strain displacement corresponding to a treading force exerted on the spindle; a sensor holding unit which defines a sensor activating zone that confronts said strain region, and which is disposed to be unmoved with said first shell-half outer wall; and a hall sensing unit including a sensor disposed in said sensor activating zone and held to be unmoved relative to said sensor holding unit, and a magnetic member disposed to be displaceable with the strain displacement of said strain region, and spaced apart from said sensor by a predetermined distance such that a magnetic field generated thereby is changed when said magnetic member is displaced with the strain displacement so as to give off a signal indicative of the magnitude of the treading force, said force measuring device characterized by a force transmitting mount which is disposed on said strain region, which extends radially toward said sensor activating zone, and which is configured to permit said magnetic member to be mounted therein.