Abstract: A detecting device is disclosed capable of measuring the rotational speed and torque simultaneously and in which the dynamic loss is reduced to the lowest level. The detecting device has a main disk, a pair of sub-detecting disks, a spring socket, a spring member, a sensor, and a display. The main disk is provided with a shaft hole centrally and with a circular recess around the shaft hole. The circular recess is further extended with a plurality of lobe grooves, each lobe groove having an adjusting groove in which an adjusting bolt is moveably disposed. An outer sub-detecting plate is fixedly attached to the main disk forming an output end, and an inner sub-detecting plate is fixedly attached t a spring socket. Teeth portions of the inner and outer sub-detecting plates are initially aligned with each other, but when the spring member is rotated a certain angle, an offset difference will be established between the inner and outer sub-detecting disks.