PURPOSE: To execute the sequence of the reading change of signals simple and clearly by relay logic, in a device for transferring signals among apparatuses.

CONSTITUTION: This interface circuit is arranged by a lst relay U for showing the condition of no signal, a 2nd relay V for holding the condition of the presence of reading change signals up to the next signal reception, a lst auxiliary relay W, which receives a signal β after excited by U excitation after reading change signals are input and which is sufficiently excited even after it is broken, a 2nd auxiliary relay X, which is excited by the relay W, receives a signal β subject to reading change and is excited during the period when it receives a normal signal γ, a control relay Y in the case when changing reading to a signal β' from the signal β, and a relay Z for holding a signal subject to reading change