This paper describes the noise suppression characteristics of an isolation transformer with an electrostatic shield in the VHF band. Several experimental results are described and the analysis of several characteristics using an equivalent circuit of the transformer is shown. The following two characteristics are investigated. They are the ratio of a differential mode output voltage to a common mode input voltage, and a common mode voltage transfer ratio. In an electrostatic shielded transformer, when a common mode current flows in the shielding plate from a primary coil, it induces a differential mode voltage in the secondary coil. We want to know the mechanism of this phenomenon in detail. Thus, we have measured how the common mode signal changes according to the number of grounding points to chassis from the electrostatic shielding plate, and the ground line impedance, etc.