A data transmission device is mounted to a wrist exerciser for detecting the rotational speed of a rotor inside the wrist exerciser and generating and transmitting exercising data associated with the rotational speed from the wrist exerciser to an external device. The data transmission device includes a transmitter unit coupled to a casing of the wrist exerciser and having a sensor for detecting the rotational speed of the rotor and issuing a detection signal, and a processing circuit receiving and processing the detection signal to provide an output of the exercising data, including counts of rotation and the rotational speed. A transmission cable has a first end connectable with the output of the processing circuit to receive the exercising data and a second end engageable with a counterpart device of the external device for forwarding the exercising data to the external device. Examples of the external devices include a personal computer, a personal digital assistant and a mobile phone having built-in memory for storage of the exercising data and a built in display for displaying the exercising data.