Abstract: The wearable heartbeat measuring device is accomplished by wearing a wrist-watch-like heart monitor device on the wrist. The heart monitor is worn on the top of the wrist with a strap around the wrist. The strap has an inflexible extending portion that extends from the heart monitor at a predetermined angle and a predetermined width with respect to the wrist so that a predetermined space is maintained between the top portion of the wrist and the strap for promoting the transfer efficiency of the pulsation force that is initially experienced at a part of the strap at the bottom of the wrist. The strap also has a flexible portion. A predetermined angle and a predetermined width of the extended portion also facilitate the flexible portion to form a flat area where the wrist contacts the strap. In the prefer embodiments, the width of the strap exceeds the width of the wrist when it is worn.