An asymmetric beta-sheet structure of spider silk is said to induce optical second harmonic generation. In this paper, using an in-house nonscanning type femtosecond pulse laser second harmonic generation microscope, we characterized the behavior of the beta-sheet of spider silk under an applied external force. The orientation of the beta-sheets was more unidirectional when the silk was extended. One of the origins of the high mechanical strength of the dragline is suggested to be the physical arrangement of its beta-sheets.