One object of the present invention is to monitor water pipes regularly and to detect water leaks by using basic equipment and to provide a device that generates an alarm when a leak occurs. The present invention responds not only to minor leaks that occur naturally because, for instance, a water pipe is decrepit, but also responds to human-caused leaks, such as those that occur because a person forgets to turn off a water faucet or because of the failure of equipment such as a hot-water heater. The present invention is a leak-detection alarm system that is composed of two main parts: (1) a leak-detecting check valve that detects movements of a movable device that makes the same movement as the valve element of a check valve that is attached to a water pipe and performs switching, and that transmits as detection signals the movement of valve elements, and (2) a signal-processing device that analyzes detection signals transmitted by the leak-detecting check valve and actuates a leak-detection alarm. The leak-detecting check valve has a main valve and an ancillary valve. The main valve operates in response to a high rate of flow of fluid through the pipe, such as occurs when there is a large leak that corresponds to said human-cause leaks, and the ancillary valve operates in response to low rate of flow of fluid through a pipe, such as occurs when there is a small leak that occurs naturally.