Heart disease has the second high mortality rate behind cancer in Japan, and requires quick treatment. To take a part in emerging mHealth, the authors developed a wearable electrocardiographic (ECG) monitoring and alerting system “iHeart”. iHeart continuously monitors patient's ECG in his/her daily activities and issues an alert to the patient as well as surrounding people if it detects abnormal heart behaviour. iHeart consists of a wireless ECG sensor and a smartphone to achieve light-weighted, low-cost system that does not degrade the patient's Quality of Life. In parallel, the authors developed ECG analysis algorithm to detect R-wave as well as arrhythmia, and implemented these algorithms in wireless ECG sensor rather than in smartphone to save power consumption of ECG sensor caused by radio communication. The authors proof the practicality and usefulness of our system in clinical experiment. This paper describes the implementation of iHeart, evaluation experiment, and future requirements of the system.