An optical reading system including a warning sound for preventing theft of a portable optical reading apparatus (scanner). A timer is activated when movement of the scanner is detected by a movement detection device mounted on the scanner, indicating that the scanner has been lifted (moved) from a stationary condition. A user determination unit determines whether the movement of the scanner is caused by an authorized user. When the user is an authorized person, a timer is reset. When the timer is not reset during a predetermined time period, a warning sound is generated at a sound level which changes from a low sound level to a loud sound level over a period of time unless a predetermined operation is performed. In one embodiment, a second warning sound is generated at a stationary receiving station when the scanner does not respond to a polling signal.