A microorganism number-measuring apparatus includes: measurement container including measurement liquid; rotary driver; bacteria-collection signal generator; measurement signal generator; output amplifier for amplifying outputs of signal generators and; I/V amplifier; impedance measuring unit for measuring impedance of liquid; microorganism number-computing unit for computing the number of microorganisms present in liquid; solution conductivity-computing unit for computing conductivity of liquid; and warm-up section for warming up at least one of I/V amplifier and output amplifier. Warm-up section computes a warm-up signal based on the conductivity computed by conductivity-computing unit. Warm-up section computes the warm-up signal having a current the same in magnitude as that flowing through measurement electrode by using the measured solution conductivity, and applies the signal to at least one of I/V amplifier and output amplifier.