A detector, comprising: a first terminal and a second terminal configured to be electrically connected to a pair of first electric wires (31A, 32A) to receive an application voltage applied between the pair of first electric wires (31A, 32A); a third terminal and a fourth terminal configured to be electrically connected to a pair of second electric wires (31B, 32B); a relay including a contact device electrically connected between the first terminal and the third terminal and configured to open and close an electric circuit between the first terminal and the third terminal; a connection part electrically interconnecting the second terminal and the fourth terminal; a voltage generation circuit configured to generate an operation voltage from the application voltage; a capacitive device electrically connected to the voltage generation circuit and configured to be charged by the operation voltage; a short-circuit detection circuit configured to detect an occurrence of short circuit between the pair of second electric wires (31B, 32B); and a control unit that operates with electric energy stored in the capacitive device, the control unit being configured to repeat a steady-state operation for detecting an occurrence of a specified event, and further configured to open the contact device of the relay in response to a detection of the occurrence of the short circuit by the short-circuit detection circuit, wherein the relay operates with the electric energy stored in the capacitive device to consume the electric energy while the short circuit occurs, and the control unit is configured to, when the short-circuit detection circuit detects the occurrence of the short circuit, open the contact device of the relay prior to a first steady-state operation which is a first one of the steady-state operation after a detection of the occurrence of the short circuit. a first terminal and a second terminal configured to be electrically connected to a pair of first electric wires (31A, 32A) to receive an application voltage applied between the pair of first electric wires (31A, 32A); a third terminal and a fourth terminal configured to be electrically connected to a pair of second electric wires (31B, 32B); a relay including a contact device electrically connected between the first terminal and the third terminal and configured to open and close an electric circuit between the first terminal and the third terminal; a connection part electrically interconnecting the second terminal and the fourth terminal; a voltage generation circuit configured to generate an operation voltage from the application voltage; a capacitive device electrically connected to the voltage generation circuit and configured to be charged by the operation voltage; a short-circuit detection circuit configured to detect an occurrence of short circuit between the pair of second electric wires (31B, 32B); and a control unit that operates with electric energy stored in the capacitive device, the control unit being configured to repeat a steady-state operation for detecting an occurrence of a specified event, and further configured to open the contact device of the relay in response to a detection of the occurrence of the short circuit by the short-circuit detection circuit, wherein the relay operates with the electric energy stored in the capacitive device to consume the electric energy while the short circuit occurs, and the control unit is configured to, when the short-circuit detection circuit detects the occurrence of the short circuit, open the contact device of the relay prior to a first steady-state operation which is a first one of the steady-state operation after a detection of the occurrence of the short circuit.