A twin roll caster including a pair of chilled rolls on which molten steel solidifies for delivery of a steel strip from a nip between the rolls, a noncontact type sensor for measuring thickness distribution of the strip widthwise of the strip, a marking mechanism that makes a mark on the strip whenever the chilled rolls make one rotation, a mark detector that senses the marks on the strip, and a nip estimating mechanism. From thickness-distribution information by the sensor and passed-mark information by the mark detector, the nip estimating mechanism calculates variation of the nip between the rolls whenever the rolls make one rotation, based on variation in thickness distribution widthwise of the strip between confirmed passing of the mark and confirmed passing of the next mark, and stores the same as data.