The president has constitutional protection against criminal action. The speaker of the National Assembly has no such device. Still, prosecutors are required to take utmost caution before initiating a criminal inquiry into a case involving the speaker, the head of one of the three separate branches of government.

In this regard, the prosecutors’ office did well when it decided to wait until Rep. Park Hee-tae resigned from his office of speaker of the National Assembly. Now that he has tendered his resignation, the prosecutors’ office will have to speed up the investigation into an allegation that Park attempted to buy votes for election to the chairmanship of the Grand National Party in 2008.

Park quit on Thursday. He is the first speaker to resign over a corruption scandal. He said he held himself, not anybody else, accountable for the case, reversing his earlier statement that he had been aware of nothing about the money-for-votes deal.

But his resignation comes a shade too late. Instead of insisting on ignorance, he could have offered to hold himself responsible and resigned on Jan. 8, the day when a lawmaker presented himself to the prosecutors’ office over his earlier remarks that he received 3 million won from one of Park’s aides. Or he could have done so when he returned from his overseas tour 11 days later.

His ill-conceived action deserved severe public censure, and all the more so, given his career. The sixth-term lawmaker started his career as a prosecutor and rose to the post of justice minister before he was elected to the National Assembly.

Now it is a matter of time until prosecutors summon him for an inquiry. But they will first have to summon Kim Hyo-jae, senior presidential secretary for political affairs, who was allegedly deeply involved in the scandal as a top aide to Park at the time.

Preferably, Kim may resign on his own volition. Otherwise, the presidential office may sack him. If no such action is taken, there will be no reason for the prosecutors to delay summoning him to question him.

What needs to be confirmed by the inquiry is how much money Park spent in violation of relevant laws and where the money came from. An allegation has already been made that Park cashed a check from a business enterprise for 50 million won before the party’s national convention was held to elect its chairman.