Research reporting significant cognitive improvement in patients with severe dementia following non-pharmacological interventions is scarce. Furthermore, few studies have focused on improvement of Mini-mental State Examination (MMSE) score in individuals with severe dementia following interventions implemented during the coronavirus 2019 (COVID-19) lockdown. COVID-19-related quarantine significantly worsened cognitive function and depression in residents of a Japanese geriatric health facility. The MMSE Japanese version (MMSE-J) scores of four participants with moderate to severe dementia (aged 88.3 ± 2.9 years) were 14.0 ± 2.2 before quarantine and 9.0 ± 1.9 one week after quarantine was started. Individualized active music therapy using Cymis (Cymis-MT), which is a new electronic musical instrument, was administered to four participants for 5 days (two sessions per day) during quarantine. Cognitive function during Cymis-MT improved significantly (P < 0.05; Cohen’s d > 2.0). MMSE-J scores at 2 and 5 days after initiation of intervention did not differ significantly from the score before quarantine. In addition, Cymis-MT significantly improved depression, as measured by the Geriatric Depression Scale, short version (Cohen’s d = 2.9). In conclusion, short-term Cymis-MT administered to non-infected patients with severe dementia during COVID-19 quarantine restored their cognitive function to pre-quarantine level. Moreover, their level of depression also improved during Cymis-MT. Therefore, Cymis-MT may be recommended as an intervention for individuals with moderate to severe dementia in seclusion situations.