[en] The magnetic properties of the EuNi₂(Si_1-xGe_x)₂ system were studied in which an intermediate-valence state of Eu is realized at around x=0.75. It was found that the compounds with 0.70≤x≤0.82 show a temperature-induced valence transition below room temperature, while those with x≥0.85 are antiferromagnetic with a stable Eu²⁺ state. A first-order valence transition induced by high magnetic field was observed for 0.75≤x≤0.82. Such valence transitions against temperature or field are discussed on the basis of the interconfigurational fluctuation (ICF) model. It is shown that the observed linear relation between the transition field and the transition temperature is explained by the present model. (author)