According to one embodiment, a lighting device includes a control circuit that includes a threshold for a case where a pair of the illumination lamps are connected in series between a positive output end and a negative output end of a power supply circuit, and a threshold for a case where one illumination lamp is connected between the positive output end and the negative output end of the power supply circuit. The control circuit determines the connected lamp number of the illumination lamps to a direct-current power supply device based on a voltage between the positive output end and the negative output end of the power supply circuit and a voltage between a non-potential connection end and the positive output end or the negative output end, and selects the threshold corresponding to the connected lamp number to control the direct-current power supply device.