A qubit generating unit generates a qubit having a predetermined quantum state. A qubit encoding unit performs quantum encoding of the generated qubit. A first pseudo-random number generating unit generates a first pseudo-random number from secretly shared information, which has been secretly shared with the quantum receiving device in advance. A quantum modulator performs quantum modulation of the qubit on which quantum encoding has been performed based on the first pseudo-random number and sends the modulated qubit to the quantum receiving device. A second pseudo-random number generating unit generates a second pseudo-random number from secretly shared information which has been secretly shared with the above quantum sending device in advance synchronously with generation of the above first pseudo-random number. A qubit demodulator performs quantum demodulation of the qubit, which has been received from the quantum demodulator based on the second pseudo-random number,