A method for adding a portable type flat panel detector to an existing computed radiography system, wherein the existing computed radiography system comprises: a radiation emitting device comprising an operation section having an operation button for starting emission of x-ray radiation; a computed radiation cassette having a fluorescent plate, wherein the existing system does not include a portable type flat panel detector; a radiation image reading device for scanning the fluorescent plate in the computed radiation cassette with an excitation beam of light and reading out a radiation image; and a console, the method comprising: adding the portable type flat panel detector to the existing radiography system, wherein the portable type flat panel detector comprises: a plurality of detection elements that convert an energy corresponding to an amount of radiation impinging on them into an amount of electrical charge; a bias line that applies a reverse bias voltage to each of said detection elements; a power supply that applies a reverse bias voltage to said detection elements via said bias line; a current detection means that detects a current flowing through said bias line; and a control means that detects a start and an end of emission of said radiation based on an increase or a decrease in the current flowing through said bias line detected by said current detection means. a radiation emitting device comprising an operation section having an operation button for starting emission of x-ray radiation; a computed radiation cassette having a fluorescent plate, wherein the existing system does not include a portable type flat panel detector; a radiation image reading device for scanning the fluorescent plate in the computed radiation cassette with an excitation beam of light and reading out a radiation image; and a console, adding the portable type flat panel detector to the existing radiography system, wherein the portable type flat panel detector comprises: a plurality of detection elements that convert an energy corresponding to an amount of radiation impinging on them into an amount of electrical charge; a bias line that applies a reverse bias voltage to each of said detection elements; a power supply that applies a reverse bias voltage to said detection elements via said bias line; a current detection means that detects a current flowing through said bias line; and a control means that detects a start and an end of emission of said radiation based on an increase or a decrease in the current flowing through said bias line detected by said current detection means. a plurality of detection elements that convert an energy corresponding to an amount of radiation impinging on them into an amount of electrical charge; a bias line that applies a reverse bias voltage to each of said detection elements; a power supply that applies a reverse bias voltage to said detection elements via said bias line; a current detection means that detects a current flowing through said bias line; and a control means that detects a start and an end of emission of said radiation based on an increase or a decrease in the current flowing through said bias line detected by said current detection means.