In a charge amplifier, an operational amplifier is provided. The operational amplifier has an inverting input terminal, a non-inverting input terminal, and an output terminal. An adjustable feedback resistor element is provide to be connected between the inverting terminal and the output terminal. A resistance adjusting circuit is provided to be electrically connected to the adjustable feedback resistor element. The resistance adjusting circuit adjusts the resistance of the adjustable feedback resistor element to a first resistance during a first predetermined period after power-on. The resistance adjusting circuit also adjusts the resistance of the adjustable feedback resistor element to a second resistance after the first predetermined period has elapsed. The second resistance is higher than the first resistance.