Since there are cases that chickens infected with highly pathogenic avian influenza (HPAI) viruses die with almost no fever, body-temperature sensing cannot be effective for the early detection of avian influenza (AI) infection in these cases. In addition, sensors directly attached to the body surface are easily affected by their surroundings, therefore it is not easy to measure the body-temperature of chickens. This paper proposes a new method to detect abnormal states of chickens with only their activity data obtained by wearable wireless sensor nodes. The method is that if its state that the present average activity of a chicken in one hour is smaller than 0.6 times minimum average daytime activity in its life continues for 3 hours, a judgment that the chicken is abnormal state is made. This method can detect the abnormal states twice as early as that with body-temperature sensing does. Since the activity sensor nodes are easily attached to chickens and hardly affected by their surroundings, this system can be reliable.