Can I be reimbursed for the costs of examination and treatment I paid out of pocket if I fall ill before completing the enrollment process with Hitachi Health Insurance?

In such cases, you will be reimbursed as long as the examination or treatment took place on or after the date of joining Hitachi Health Insurance(Qualification Date ･ Dependency Recognition Date）.

Pay the costs up front yourself, then apply to the Health Insurance Society for reimbursement of the portion covered by insurance.