It is assumed that the step count (N13) of a livestock animal (A1) in a time slot (t2 to t3) is greater than or equal to a threshold value. An estrus notifying apparatus (102) calculates a degree of reduction of the step count of other livestock animals (A2 to An) in the same herd for a previous time slot. The degree of reduction is a value indicative of a level of reduction of a measurement value (N) of the livestock animals (A2 to An) for the previous time slot (t1 to t2) from a measurement value (N) for a time slot (t0 to t1) before the previous time slot and is a reduction rate, for example. It is assumed that, for example, the step counts of the livestock animals (A2 to An) are half or less for the time slot (t1 to t2) relative to the time slot (t0 to t1). The estrus notifying apparatus (102) considers that the measurement value becomes less than the threshold value due to eating even though the livestock animal (A1) is in estrus in the time slot (t1 to t2), and determines that the livestock animal (A1) is in estrus during the time slot (t1 to t2). The estrus notifying apparatus (102) outputs information indicating that the estrus of the livestock animal (A1) started in the time slot (t1 to t2).