Fibromyalgia (FM) is a health burden due to its high prevalence, negative impact on patients' quality-of-life and lack of effective therapies. Given the primary symptom in FM patients is widespread spontaneous pain, the pain measurement using indicator reflecting patients' spontaneous pain should be implemented in the preclinical research for FM. The present study applied the rat grimace scale (RGS), coding of facial expressions, to the reserpine-induced myalgia (RIM) rat, a well-validated animal model of FM. Animals were videotaped by two high-resolution video cameras. Still images of the animal face were then captured. RGS scoring was conducted according to the method described by Sotocinal et al (2011). The RIM rat exhibited a long-lasting increase of RGS score. Time-course of the change of RGS score was compared to that of paw withdrawal threshold measured by von Frey hair filament, a conventional method for evoked pain. The elevated RGS score showed varied responses to clinically used analgesic drugs. The present study suggests that RGS score in the RIM rat simulates spontaneous pain in FM patients. This pain measurement scheme would contribute to the prediction of efficacies of analgesic therapies for FM patients.