The invention relates to an alternating magnetic field application device for heating magnetic or magnetizable substances in biological tissue, in particular for thermal therapy using magnetic nanoparticles, composed of a large applicator (1) having a magnetic yoke (2) and two oppositely situated pole shoes (7, 8) on the magnetic yoke (2) which are separated by an exposure gap (13), and having two magnetic coils (9, 10), which are respectively associated with a pole shoe (7, 8), for generating a substantially homogenous alternating magnetic field (12) of a given field strength in the exposure gap (13), wherein the biological tissue to be irradiated may be brought into the exposure gap (13) as an exposure target volume. According to the invention, in the exposure gap (13) a field concentrator (19) is situated in the immediate proximity of the biological tissue to be irradiated as the exposure target volume, in particular on or in a patient in the immediate proximity of a body part to be irradiated, such as a diseased prostate (23), the field concentrator concentrating the alternating magnetic field (12) of the large applicator (1) in the target volume and thereby locally enhancing the alternating magnetic field at that location.