of luminance signals from the side of the melt to the side of the monocrystalline rod along a sensing line intersecting the crystallization interface thereby successively outputs a series of sampled luminance signals determines a value as a reference level relative to this specific level of the luminance signals e g a peak value thereof and discriminates the crystallization interface when one of the series of sampled luminance signals exceeds the reference level a pixel position corresponding to this discrimination is determined as a pixel position corresponding to the crystallization interface application_number US 18589488 A description an embodiment of the present invention will be described below with reference to the accompanying drawings the hardware configuration of a device for detecting the position of a crystallization interface will first be described with reference to fig2 components which are identical to those shown in fig8 are indicated by the same reference numerals a composite image signal is supplied from an industrial tv camera 18 to a sync separator circuit 24 and a horizontal synchronizing signal hsync a vertical synchronizing signal vsync and a luminance signal s are separated respectively from the composite image signal the luminance signal is periodically sampled by a sample and hold circuit 26 so as to hold values of the level of this signal the values thereby sampled and held are supplied to an a d converter 28 are then converted into digital values and are read by a microcomputer 30 thereby detecting the position of a crystallization interface 20 in this embodiment as shown in fig7 the position at which the sampling is started is set to a scanning line which is located m lines before the scanning line.