Quantum key distribution (QKD) is an amazing quantum communication protocol which realizes unconditionally secure random bit sharing. However, when we implement the standard QKD system, we have to use an intractable quantum device such as a photon detector. In order to avoid such difficulties, continuous variable QKD (CV-QKD) is invented, and it is known that CV-QKD is also unconditionally secure. When we implement the CV-QKD, we use a homodyne detector or a heterodyne detector which can be realized by a tractable device such as a PIN photodiode. When we use the tractable device, sender and receiver have to prepare laser sources (LO) individually, and these laser sources must be synchronized. However, the synchronization of the spatially separated LO is very difficult. Therefore, when we demonstrate the CV-QKD, a strong reference light made from a LO made by the sender is transmitted into the receiver, and the transmitted strong light is used as a sabstitute for the LO of the receiver. In such a situation, i.e. the situation that the strong reference light can be manipulated by eavesdropper (Eve), unconditional security is not ensured. We investigate this situation, and show that when the photon number of the reference light is sufficiently large, the CV-QKD in the situation is also unconditionally secure in spite of the manipulation of the reference light by Eve. In this seminar, we show an outline of the proof.