question: BACKGROUND: Self-isolation is an important personal protective measure in inhibiting the transmission of coronavirus disease 2019 (COVID-19) as people carry out economic and social activities amid its spread. Yet few studies have clarified the actual implementation status of self-isolation during an outbreak. This study aimed to reveal the actual implementation of self-isolation among Japanese workers during the COVID-19 outbreak and the factors inhibiting this measure. METHODS: This was a cross-sectional study based on an internet survey completed by 1,226 workers (60.0% men) living in 7 prefectures (i.e., Tokyo, Kanagawa, Saitama, Chiba, Ibaraki, Tochigi, and Gunma) who were selected among registrants of an Internet research company, between May 12 and 17, 2020. Participants were asked whether they had experienced fever or other cold symptoms between February 17, 2020 and the date of the survey. Those who responded affirmatively were asked where they had visited (e.g., hospital, work, and shopping for groceries or necessities) to clarify whether they had left the house within 7 days after symptom onset. We performed multivariate logistic regression analysis to clarify the relationship between going to work within 7 days after symptom onset and both sociodemographic factors and employment-related constraints. RESULTS: Of the survey participants, 82 had experienced fever or other cold symptoms (6.7%). Among these participants, 51 (62.2%) went to work within 7 days after symptom onset. A mere 17.1% practiced strict self-isolation. Multivariate logistic regression analysis revealed that those living outside the metropolitan area (i.e., Ibaraki, Tochigi, and Gunma), working as a company employee, and being unable to work from home were associated with going to work within 7 days after symptom onset. CONCLUSIONS: The prevalence of strict self-isolation among participants who experienced cold-like symptoms during the COVID-19 outbreak was extremely low, and 62.2% of these participants went to work within 7 days after symptom onset. This study highlights the need for further public awareness regarding self-isolation and countermeasures against factors that obstruct it.