A game device includes a processor, a display device having a screen displaying a plurality of enemy vessel images, an invisible vessel image and a target-finding frame, and a joystick device. The enemy vessel images and the invisible vessel image are moving across the screen. The invisible vessel image is concealed in the screen. The joystick device controls movement of the target-finding frame in the screen. The processor is capable of transforming the invisible vessel image into a visible vessel image when the target-finding frame is moved to overlap the invisible vessel image. In case the enemy vessel images and the invisible vessel image are attacked, the processor executes in such a manner to blow the enemy vessel images and the invisible vessel image off the screen.