When calculating the insured period as a category 1 insured person, the insured period as a category 2 insured person, or the insured period as a category 3 insured person, a month in which there was a change in the category of insured person (meaning the distinction as to whether a person is a category 1 insured person, a category 2 insured person, or a category 3 insured person; the same applies hereinafter) is deemed to be a month during which the person was an insured person of the category after the change. When there have been changes in the category of insured person two or more times within the same month, that month is deemed to be a month during which the person was an insured person of the last category.