Abstract: A body fat thickness measuring device comprising a pair of infrared emitting diodes, one emitting a steady, low-intensity light, and the other emitting periodic, high-intensity pulses of light, to be placed against the skin where the fat is to be measured. The device contains an array of detectors in the form of infrared-sensitive photo-transistors which are placed against the skin in predetermined locations near the two diodes, yet shielded from ambient light. The detector array provides signals proportional to the amount of infrared light detected, and these signals are summed and amplified, forming a composite signal. The amplitude of this composite signal, which is indicative of the thickness of the layer of fat, is displayed on a digital readout device.