(Accounting Standard for Retirement Benefits 3,2)
In principle, past service costs are recognised as expenses, and are amortised over a fixed period
(within the period of the remaining average service lives).
The amortisation periods for past service costs and for actuarial gains and losses may be determined separately.
For actuarial gains and losses, amortisation may commence from the period following the period in which they arose, however this is not allowed for past service costs.
(Accounting Standard for Retirement Benefits Note 11)
Past service costs relating to retired employees can be distinguished from other past service costs and can be expensed in full when they arise.