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The internal electric field due to the piezoelectric effect in an InGaN/GaN multiple quantum-well structure is measured via photoluminescence (PL), electroreflectance (ER), and photocurrent (PC) spectroscopies, and the measurement results are compared with each other and with theoretically calculated values. The flat-band voltage is estimated by measuring the applied bias voltage that induces the maximum in PL peak energy by the quantum-confined Stark effect, 180° phase shift in the ER spectrum, and the smallest band tail state in the PC spectrum. The internal electric fields estimated by the PL, PC, and ER spectra are -1.81, -2.12±0.14, and -2.04 MV/cm, respectively. The measured piezoelectric fields are in good agreement with theoretically calculated values. Possible factors affecting piezoelectric field measurements are discussed from various perspectives.