The apparatus includes: At least one light-emitting, optoelectronic, functional element, which, during operation, activated by an electrical signal, emits light, at least at times; at least one light-sensitive, optoelectronic, functional element, which, during operation, activated by light falling thereon, delivers, at least at times, an electrical signal; and at least a third, light-conducting, functional element composed at least partially of transmissive material, especially translucent plastic or glass. The third functional element conveys, during operation, light coupled thereinto, and has at least one essentially planar boundary surface, which deflects light conveyed in the apparatus. The at least one, essentially planar, light-deflecting, boundary surface has a surface normal, which is inclined at least with respect to the principal ray axis at least of one of the two optoelectronic, functional elements, with an imaginary principal ray axis of the functional element representing a direction of maximum intensity of the light emitted therefrom during operation and an imaginary principal ray axis of the second functional element representing a direction of its maximum sensitivity to light incoming during operation. The apparatus of the invention is distinguished by a very simple construction, with a relatively high light coupling factor, and is especially suited for use as an optocoupler for field devices of industrial measurements and automation technology.