Abstract: An apparatus for precisely detecting the diameter of the single-crystal material which is fabricated by a continuously charged method is disclosed. Variables such as the crucible raising amount, the material charging amount and the weight of a growing single crystal can be detected by conventional detecting apparatus. A present melt surface position is obtained by initial melt surface position+the crucible raising amount +(material charging amount/crucible area)-(weight of the single-crystal/crucible area) and then provided to a diameter control apparatus. The sensing angle with respect to the initial melt surface position 3a is .theta.. The height from initial melt surface position 3a to one-dimensional image sensor 2 is h. The horizontal distance between the scanning line and the one-dimensional image sensor is r. When the melt surface falls by a distance of a .DELTA.h from position 3a to position 3b, the adjusted sensing angle .theta.' can be obtained from .theta.'=cos.sup.-1 [(h+.DELTA.h)/{(h+.DELTA.h).