A reception device includes at least a light receiving unit, a phase calculation unit, and a demodulation unit. The light receiving unit receives intensity-modulated light from a transmission device. The transmission device executes phase modulation to a bit stream as an object to be transmitted and emits the intensity-modulated light which has an intensity changing at a preset time cycle and a phase changing in response to the phase modulation, and the phase is maintained during M cycles. The phase calculation unit detects the intensity of the intensity-modulated light p times per cycle of the intensity-modulated light and repeatedly calculates the phase of the intensity-modulated light on the basis of a result of the detection. The demodulation unit executes demodulation, which corresponds to the phase modulation, to the phase calculated by the phase calculation unit, and generates transmitted bit stream.