A system comprising: a refrigeration cycle comprising a piping unit for circulating a refrigerant in a refrigeration device or air conditioning device, and a condenser, an expansion valve, an evaporator, and a compressor that are connected using the piping unit, the refrigerator cycle being configured so that the refrigerant circulation through the piping unit flows to the condenser, the expansion valve, the evaporator, and the compressor in the stated order; and a device for detecting a refrigerant leak, the device being configured to detect and report a leak of refrigerant in the refrigeration cycle, wherein the device for detecting a refrigerant leak comprises an ultrasonic wave transmitter for transmitting ultrasonic waves, an ultrasonic wave receiver for receiving the ultrasonic waves transmitted by the ultrasonic wave transmitter, an ultrasonic wave reception determination unit, and a leak reporting unit, wherein the ultrasonic wave transmitter and the ultrasonic wave receiver are provided, in a state of mutual opposition, to the outer surface of the piping unit, characterized in that the ultrasonic wave transmitter and the ultrasonic wave receiver are provided at positions in the refrigeration cycle downstream from the condenser and upstream from the expansion valve, wherein the ultrasonic wave transmitter is configured to transmit ultrasonic waves having a frequency of 3 MHz or less, and wherein in a state in which the refrigerant is circulating between the mutually opposed ultrasonic wave transmitter and ultrasonic wave receiver, the ultrasonic waves are transmitted from the ultrasonic wave transmitter toward the ultrasonic wave receiver, the ultrasonic wave reception determination unit is configured to determine whether the ultrasonic wave receiver has received the ultrasonic waves transmitted by the ultrasonic wave transmitter, and the leak reporting unit is configured to externally report a leak event when the ultrasonic wave reception determination unit has determined that the ultrasonic wave receiver has not received the ultrasonic waves transmitted by the ultrasonic wave transmitter. a refrigeration cycle comprising a piping unit for circulating a refrigerant in a refrigeration device or air conditioning device, and a condenser, an expansion valve, an evaporator, and a compressor that are connected using the piping unit, the refrigerator cycle being configured so that the refrigerant circulation through the piping unit flows to the condenser, the expansion valve, the evaporator, and the compressor in the stated order; and a device for detecting a refrigerant leak, the device being configured to detect and report a leak of refrigerant in the refrigeration cycle, wherein the device for detecting a refrigerant leak comprises an ultrasonic wave transmitter for transmitting ultrasonic waves, an ultrasonic wave receiver for receiving the ultrasonic waves transmitted by the ultrasonic wave transmitter, an ultrasonic wave reception determination unit, and a leak reporting unit, wherein the ultrasonic wave transmitter and the ultrasonic wave receiver are provided, in a state of mutual opposition, to the outer surface of the piping unit, characterized in that the ultrasonic wave transmitter and the ultrasonic wave receiver are provided at positions in the refrigeration cycle downstream from the condenser and upstream from the expansion valve, wherein the ultrasonic wave transmitter is configured to transmit ultrasonic waves having a frequency of 3 MHz or less, and wherein in a state in which the refrigerant is circulating between the mutually opposed ultrasonic wave transmitter and ultrasonic wave receiver, the ultrasonic waves are transmitted from the ultrasonic wave transmitter toward the ultrasonic wave receiver, the ultrasonic wave reception determination unit is configured to determine whether the ultrasonic wave receiver has received the ultrasonic waves transmitted by the ultrasonic wave transmitter, and the leak reporting unit is configured to externally report a leak event when the ultrasonic wave reception determination unit has determined that the ultrasonic wave receiver has not received the ultrasonic waves transmitted by the ultrasonic wave transmitter.