Abstract: An ovulation period detecting apparatus for mammals is provided comprising: a bar-shaped detecting unit to be inserted in the vagina of a mammal; electrodes arranged on the bar-shaped detecting unit so as to be in contact with the vaginal mucous membrane when the bar-shaped detecting unit is inserted in the vagina of the mammal; pressure detecting means for detecting a pressure at which the electrodes are pressed against the vagina of the mammal; impedance detecting means for applying a voltage across the electrodes and detecting the value of impedance between the electrodes when the pressure detecting means detect a pressure equal to or greater than a prescribed pressure over a prescribed time; and data outputting means for outputting the value of the impedance detected by the impedance detecting means.