Imaging signal obtained through exposure for divided exposure times is A/D converted to digital imaging signal. Dark current component is subtracted from the digital imaging signal. The result of subtraction is accumulated and stored in a first memory. Next, exposure for divided exposure times is performed with the imaging device shielded from light. The obtained imaging signal is A/D converted to digital imaging signal. Dark current component is subtracted from the digital imaging signal. The result of subtraction is accumulated and stored sequentially in a second memory. The digital imaging signal stored in the second memory is subtracted from the digital imaging signal stored in the first memory. Then the result of subtraction is output. The word length allocated to one pixel in the first and second memories is longer than the word length of one A/D converted pixel.