Disclosed here is an active device for monitoring animal being capable of measuring vital signs of the animal and actively signaling an external apparatus. The device comprises a pin adapted to penetrate a predetermined portion of the animal； a gasket having two ring portions formed as one piece； a base having a first sleeve portion for allowing the pin to squeeze through so as to be engaged with each other and blocking the pin from slipping off after engagement； a monitoring unit； and a cover having a second sleeve portion to receive the first sleeve portion, and being joined with the base in a waterproof manner so as to accommodate the monitoring unit therein.
