A device for determining the body core temperature of a living being includes a first temperature sensor (15) for detecting the skin temperature T1, an insulator (11) for receiving the first temperature sensor (15), a second temperature sensor (17) in the insulator (11) for detecting a temperature T2 and a third temperature sensor (19), which is arranged opposite the first temperature sensor (15) at the insulator (11), for detecting a temperature T3 near the environment. Assuming a constant ratio of lateral heat fluxes α at the insulator (11) between the temperature sensors (15, 17, 29), the body core temperature Tcore is determined by the relationship described by the formulaTcore=T1·[1+kskg(1+1α)]-T2·(kskg+ks+ktαkg)+T3·ktαkg in which kg=heat transfer coefficient of the tissue and ks, kt=heat transfer coefficients of the insulator (11).
