A 41-year-old man visited our clinic because of headache with fever, suggestive of aseptic meningitis. His headache improved in a few days. His neurological examination showed positive jolt accentuation and myoclonus of the thoracoabdominal muscles extending to extremities upon patellar tapping. His myoclonus had been occurring spontaneously from early adolescence, especially in relaxed states such as drowsiness. The myoclonus was not triggered by tactile, auditory, or visual stimulation. Polymyography revealed that the myoclonus originated around the T4 spinal level and slowly propagated both upward and downward. These findings were indicative of spontaneous and reflex propriospinal myoclonus (PSM). No abnormalities were seen on brain and spinal MRI. Furthermore, the amplitude of the cortical component of the somatosensory evoked potential (SEP) after electrical stimulation of the tibial nerve was enlarged bilaterally. It was speculated that the ascending signals from the myoclonus generator at T4 to S1 may have modulated the excitability and inhibitory function of S1 in this patient. This report may be the first case of idiopathic PSM accompanied by giant SEP.