An encryption method of a 3-D object based on the phase modulation of an object wave is proposed. The phase of the object wave is modulated by a virtual optical random phase mask using a digital holographic technique. The keys of an encryption step are both a phase distribution and a position of the virtual optical phase mask. If either of them is not correct, we cannot decrypt the 3-D object. Owing to a characteristic of a hologram, we can see some parallax of a reconstructed 3-D object. Experimental results are presented to confirm the proposed method.