The spectral linewidth Δv of a vertical cavity surface-emitting laser was measured for the first time. The linewidth measured by a delayed self-homodyne method was 50 MHz at an output power of 1.4 mW under room-temperature CW operation. The linewidth we obtained was quite narrow in spite of the short cavity configuration of the SE laser. This narrow linewidth is attributed to the high-reflectivity mirrors. The measured linewidth is in good agreement with theoretical values.