An object of the present invention is to provide a cell sheet embedding agent, which makes it possible to stably transport a cell sheet at the time of transport at a low temperature and to collect the cell sheet from the cell sheet embedding agent in a simple manner after being transported, a cell sheet-containing composition, and a kit. According to the present invention, there is provided a cell sheet embedding agent containing a polypeptide represented by the following Formula 1, in which in a case where a molecular weight distribution of the polypeptide is measured, a peak area of a maximum molecular weight is equal to or greater than 80% of the total peak area of all molecular weights. A-[(Gly-X—Y)n]m—B  Formula 1: In the formula, X and Y each independently represent an amino acid, m is an integer of 2 to 10, n is an integer of 3 to 100, and A and B each represent any amino acid or any amino acid sequence.