A Y-branch digital optical switch is fabricated using proton exchange on a Z-cut $LiNbO_3$ substrate. With no applied voltage the power division ratio between two output branches is about 52:48 and the lateral and vertical output mode sizes are 9.6㎛ and 5.1㎛, respectively. The fabricated digital optical switch has a crosstalk of -22dB and an extinction ratio of -23dB at applied voltage of 16V.