Mich. Comp. Laws § 201.1: Resignation from public office; to whom made.

Sec. 1.

Resignations shall be made as follows:

By officers appointed by the governor alone, or by the governor by and with the advice and consent of the senate, or both branches of the legislature, to the governor;

By all other officers holding their offices by appointment, and not by election, to the body, board, or officer that appointed them;

By all other officers holding their offices by election, to the governor, if the governor is authorized to fill the vacancy created by the resignation.