The principle of time priority is ignored such as orders placed before the opening price is determined during the morning and afternoon trading sessions. That is, all orders placed before the opening price is determined are regarded as simultaneous orders.
(All orders placed during morning session and Zaraba, orders when opening price of the day isn't determined or orders trade price isn't determined after the Trading Halt, orders are regarded as simultaneous orders.)
In this case, the orders are executed by the "Itayose method".

(Example)
If the opening price is determined at 9:03, all orders has been placed by 9:03 will be regarded as simultaneous orders.

Trading Rules of Domestic Stocks is posted on the JPX website.
URL: https://www.jpx.co.jp/english/equities/trading/domestic/04.html