PURPOSE: To obtain a device with high reliability and exceedingly simple structure by providing an optical cutoff detection type photoelectric mechanism with two detection optical axes or more in parallel to one another in the cross- sectional direction of a funnel neck.

CONSTITUTION: The detection optical axis 231 of a first optical cutoff detection type photoelectric switch mechanism consisting of a projector 211 and a photodiode 221 and the detection optical axis 232 of a second optical cutoff detection type photoelectric switch mechanism consisting of a projector 212 and a photodiode 222 are opposedly in the cross-sectional direction of a funnel neck 4 so as to be parallel to each other and the distance C between the detection optical axes 231 and 232 is half the sum of outer and inner diameters D and d of the funnel neck 4 to be detected. When such detection optical axes 231 and 232 intersect the funnel neck 4, the optical sensors in both photodiodes 221 and 222 are operated simultaneously and set the output to zero. As a result, the funnel neck with the funnel neck diameter to be detected is assumed to pass through.