Provided are an Mn--Zn ferrite in which the core loss is low and Bs is high at high temperatures; a transformer capable of being small-sized and suitable to use at high temperatures; and a method for efficiently driving the transformer. Also provided is a transformer capable of being small-sized and suitable to use in a broad temperature range including high temperatures. Further provided are a high-efficiency transformer capable of being small-sized, in which the core loss in the ferrite core is low and the saturation magnetic flux density is high therein at the temperature at which the transformer is driven; and a high-efficiency driving method for the transformer. Depending on its use, Mn--Zn ferrite to be the ferrite core for the transformers comprises, as the essential components, specific amounts of Fe.sub.2 O.sub.3, ZnO and MnO, and, as the side components, specific amounts of SiO.sub.2, CaCO.sub.3, Nb.sub.2 O.sub.5 and ZrO.sub.2. The core of the transformers is made of the Mn--Zn ferrite.