The present invention relates to a method of updating an elevator control device and a main controller unit. The updating method includes a dismounting process of detaching the existing main controller substrate from the casing of the control device, wherein the existing main controller substrate is connected to the casing of the elevator control device, and has a first communication interface andoutputsa predetermined form of signal to control the elevator through the first communication interface; and a first mounting process of mounting the main controller unit to which the new main controller substrate and the connection conversion substrate are connected in the casing of the control device at a position where the existing main controller substrate is removed, wherein the new main controller substrate is smaller in size and different in signal form than the existing main controller substrate. The connection conversion substrate is the same as the existing main controller substratein size, and has the function of receiving the signal output from the new main controller substrate and converting the signal into a signal of a predetermined form, and has a second communication interface that can communicate with the signal of the predetermined form and has the same standard as the first communication interface.