An elementary formulation of Kubota's proof of Satz 4 about biquadratic number fields

The aim of this note is to give an elementary formulation of Tomio Kubota’s proof of Satz 4 in his paper in Nagoya Math. t. 10 (1956) since this proposition is a cornerstone for the computation of the order of the Polya group of the biquadratic number fields.