A force measuring device for a bicycle includes a sleeve shell having a first shell-half outer wall and an inner tubular wall which are disposed between a bottom bracket shell and a spindle, a ring body disposed on the inner tubular wall and 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 defining a sensor activating zone and a hall sensing unit including a sensor which is disposed in the sensor activating zone, and a magnetic member which is displaceable with the straining movement to change a magnetic field generated thereby so as to give off a signal indicative of the magnitude of the treading force.