The invention provides a device and system for measuring core (11) body temperature, comprising two pairs of temperature sensors (8-1a, 8-1b, 8-2a, 8-2b), with a structure (2, 3, 4, 5, 6, 7) therebetween, and a heat flux modulator (9) for changing the heat flux through one pair (8-1a, 8-1b) more than the heat flux through the other pair (8-2a, 8- 2b). By measuring the temperatures for the two pairs of temperature sensors, the core (11) body temprature may be derived. This device allows more design freedom, and it is easier to manufacture and gives a more accurate core temperature.