
Sonthi informed CNS office on Sunday and his resignation took effective on Monday after CNS secretary general was informed of his decision, the spokesman said.
CNS's deputy chairman, ACM Chalit Pukpasuk, also head of the airforce, will become acting CNS chief.
"Gen Sonthi has stated in his resignation letter that he decided to resign because he already completed his mission as CNS chief," the spokesman said.
"Gen Sonthi said he decided to resign so that he could do other job on security matter," he said.
Sansern declined to comment on speculations that Sonthi resigned so that he could become deputy prime minister on security in Surayud government.
Meanwhile Army Commander in Chief Gen Anupong Paochinda said Gen Sonthi telephoned him on Monday, saying that he already resigned.
Anupong said he did not ask his reason and whether he would become a deputy prime minister or not.
"Gen Sonthi will explain why he decided to resign on Tuesday which is his 60 th anniversary at his Kesakomon residence," Anupong said.