A material determining apparatus for a steel product comprising: an imaging device for continuously imaging a spark generated during rubbing the steel product at a plurality of times; a detecting part for detecting spark regions and bursting spark regions having three or more end portions among the spark regions, from each of the imaged pictures imaged by the imaging device; a calculating part for calculating a total of the spark regions and a total of the bursting spark regions by summing up the numbers of the spark regions and the bursting spark regions detected by the detecting part with regard to each of all the imaged pictures, so as to calculate a ratio of the total of the bursting spark regions with respect to the total of the spark regions; and a determining part for determining a carbon content of the steel product based on the ratio; characterised in that the detecting part is adapted to carry out the following steps: a first detecting step of detecting a spark candidate pixel group constructed by pixels which are continuous along one pixel line among pixel lines constructing each of the imaged pictures imaged by the imaging device, and which have gray levels that are equal to or more than a first threshold value; a second detecting step of detecting a maximum gray level among the gray levels of the pixels constructing the spark candidate pixel group, and binarizing the pixels constructing the spark candidate pixel group by a second threshold value which is less than the maximum gray level and is more than the first threshold value, and which has a predetermined ratio with respect to the maximum gray level, thereby detecting the pixels constructing a spark pixel group from the pixels constructing the spark candidate pixel group; and an identifying step of preparing a binarized image expressing the spark pixel group for each of the imaged pictures by executing the first detecting step and the second detecting step with respect to all the pixel lines constructing each of the imaged pictures, and identifying the spark pixel groups which are continuous with any direction in the each of the binarized images as the spark region. an imaging device for continuously imaging a spark generated during rubbing the steel product at a plurality of times; a detecting part for detecting spark regions and bursting spark regions having three or more end portions among the spark regions, from each of the imaged pictures imaged by the imaging device; a calculating part for calculating a total of the spark regions and a total of the bursting spark regions by summing up the numbers of the spark regions and the bursting spark regions detected by the detecting part with regard to each of all the imaged pictures, so as to calculate a ratio of the total of the bursting spark regions with respect to the total of the spark regions; and a determining part for determining a carbon content of the steel product based on the ratio; characterised in that the detecting part is adapted to carry out the following steps: a first detecting step of detecting a spark candidate pixel group constructed by pixels which are continuous along one pixel line among pixel lines constructing each of the imaged pictures imaged by the imaging device, and which have gray levels that are equal to or more than a first threshold value; a second detecting step of detecting a maximum gray level among the gray levels of the pixels constructing the spark candidate pixel group, and binarizing the pixels constructing the spark candidate pixel group by a second threshold value which is less than the maximum gray level and is more than the first threshold value, and which has a predetermined ratio with respect to the maximum gray level, thereby detecting the pixels constructing a spark pixel group from the pixels constructing the spark candidate pixel group; and an identifying step of preparing a binarized image expressing the spark pixel group for each of the imaged pictures by executing the first detecting step and the second detecting step with respect to all the pixel lines constructing each of the imaged pictures, and identifying the spark pixel groups which are continuous with any direction in the each of the binarized images as the spark region.