time from the peak 41a to the peak 41b be a third value the first and second values correspond to the amount of the received energy and the third value corresponds to the length l detected in the direction of arrow a in which the package w is advanced any difference between the first and second values indicates that the bead heights h are not uniform non difference indicates that the heights h are the same after receiving data about the condition of beads in the form of differential waveform signals the comparator 5 compares each of the first second and third values with the upper limit and the lower limit to see whether or not they fall within each reference range each of the first second and third reference ranges represents a 34 normal 34 zone in which the beads may be considered to be in normal condition in this way when the first value falls within the first reference range the height h 1 may be evaluated as 34 normal 34 likewise when the second value falls within the second reference range the height h 2 may be evaluated as 34 normal 34 when the third value falls within the third reference range the length l of the bead w in the advancing direction thereof may be evaluated as 34 normal 34 depending upon the packages all of the results can be adopted or else one or two of the results can be adopted so as to see whether the beads is in normal condition or not the amplified signals 31 are transformed into differential waveform signals 41 thereby eliminating the necessity of using a chopper which otherwise would be.