An apparatus for charging a portable device battery with a vehicle battery is provided. The surface hardness of the core has a JIS-C hardness value of 40 to 95, the center hardness of the core has a JIS-C hardness value of 30 to 72, and the hardness difference therebetween is from 4 to 14. Each row has a left end and a right end, and each column has a top end and a bottom end. This allows for an increase in the voltage between both terminals of the electret capacitor, and thus suppresses that the sensitivity of the microphone unit is lowered due to the parasitic capacitor.