Abstract: A detection device which, when an arrhythmia occurs, can prompt rapid taking of a countermeasure against the arrhythmia, and an alarm system having the detection device are provided. A portable detection device 10 has: a measuring section which can measure an electrocardiogram in a state where the section is applied to the body surface of the user; an analyzing section which analyzes whether the electrocardiogram contains an abnormal waveform indicating an arrhythmia or not; and a transmitting section which, if the analyzing section detects the abnormal waveform, transmits a detection signal indicating that the abnormal waveform is detected, to a communication terminal 20 around the user.