The authors have carried out measurement of a maximum length of transverse coherent separation by means of spin precessing neutrons and spin splitters. We have measured the spin precession when the transverse separation is up to 4.0 pm using a spin interferometer in MINE in JAERI. We interpreted that the loss of the visibility of the spin interference is caused from less overlap of coherent probability distribution of up arrow and down arrow spin neutron waves. Furthermore we have installed a neutron spin interferometer at ULS in JAERI in order to research the existence of an intrinsic maximum coherent separation. The results shows the feasibility of the measurement of the coherent separation with a well monochromatic beam (C) 1999 Elsevier Science Ltd. All rights reserved.