In the method of producing ditrimethylolpropane of the present invention, the distillation still residue obtained in the production of trimethylolpropane is subjected to crystallizing treatment while strictly regulating the amount of organic solvent to be used, the crystallization temperature and the crystallization time within the specific ranges. The burner includes a retention plate at an output end of a gas supply line having a central hub from a which a plurality of spokes radially extend. Furthermore, the method includes generating a Multiple Attack Vector graph based on the information gathered and one or more scan parameters. The second sheet feeding tray has a pair of side guides which are provided in parallel to the sheet feeding direction and which contact against side ends of the sheet of the second side to guide the sheet of the second side to the print part. In order to reduce the volume and weight of such a membrane-type coupling, the input membrane pack is disposed between the two output membrane packs. The elastically deformable element permits to create a bone cavity in a regular and uniform way.