A locking device for an automobile tray is disclosed. The locking device includes an inner housing having a front panel formed with a holding recess, an outer housing internally defining an operating space for sliding movement of the inner housing therein, a mount member having a bottom opening for communicating with the holding recess and a front operating opening, a knob partially protruded outward through the operating opening of the mount member, the knob being linearly moved into the operating opening by the user's operation, a rotating member having a hinge shaft connected to the mount member so as to be rotated by the knob, a first elastic member having one end supported by the mount member and the other end supported by the rotating member, to provide the rotating member with an elastic restoration force so as to return the rotating member to its original position, a latch member configured to be caught by one end of the rotating member, the latch member being moved upward by the rotating member so as to be released from the holding recess, and a second elastic member installed between the latch member and the mount member and adapted to provide the latch member with an elastic restoration force so as to allow the latch member to be returned downward through the bottom opening of the mount member.