The structures of water at the interfaces between a fused quartz surface modified by a self-assembled monolayer (SAM) of an amino-terminated silane molecule and electrolyte solutions of various pHs were investigated using sum frequency generation (SFG) vibrational spectroscopy. The structure of the interfacial water significantly depended on the solution pH. The effect of the electric double layer generated by the protonation of the amino groups in the SAM and unreacted silanol groups on the quartz substrate is considered to be responsible for the pH-dependence.