Electrostatically induced voltage generated by electrostatic induction causes malfunction and eventual failure of electronic apparatus. This type of vo1tage is frequently produced in a partially open metal box when a charged body such as a charged human body or device is present near the given electronic apparatus. However，it is difficult to measure the voltage induced in the metal box without making electrical connections between conductors in the box and measurement instruments. The author has successfully measured the induced voltage in the metal box by using a sphere gap along with an electromagnetic wave sensor. In this study，the relationship between the induced vo1tage and a ratio of the width of the shielding conductors to that of the opening of the metal box is investigated. The resu1t shows the induced vo1tage generated in the metal box decreases as the ratio increases. In the experimental condition，the induced vo1tage becomes less than 0.1% of a voltage of the charged body when the ratio becomes more than 80%. The results of this study are expected to be useful in designing electronic apparatus and preventing their malfunction and failure due to electrostatically induced vo1tage.