Quantum phase noise of a single longitudinal mode AlGaAs semiconductor laser is measured by converting it into intensity noise. A Michelson interferometer is used and a time delay is introduced. Results on spectral linewidth and noise spectrum are in good agreement with theoretical analyses based on a rate equation. Spectral linewidth of the semiconductor laser decreases from 100 MHz to 4 MHz in proportion to (I/Ith−1)−1 at the bias level of I/Ith=1.05 − 1.4.