In this letter, we propose a novel scheme for all-optical logic gate using a single-mode Fabry-Perot laser diode (FP-LD). Modulation of the self-locking state of a single-mode FP-LD by injecting an input pump beam is the basic operation principle of the proposed logic gate. The proposed scheme does not need any external probe light source that is required in other schemes using multimode FP-LD. The operation principle is explained and the experimental results of NOR and NOT functions at 10 Gb/s are presented. The ON-OFF contrast ratio of over 40 dB is measured and an error-free gate operation is achieved.