Pulp injection molding (hereafter PIM) is a new molding technology for realizing 3-dimensional molded products made of paper-based materials composed mainly of pulp and starch. It was developed with the purpose of reducing environmental impact. In this study, the authors proposed a new color-marking method which extracts, as flow history, information about the location on molded products of colored tracers initially positioned on the cavity surface, and built a flow model clarifying the material filling behavior using a rectangular cavity (1 mm thick) with an area varied in thickness (made up of rib and groove),intermediately positioned perpendicular to the flow.
(1)Flow behavior of area whose thickness can be increased (rib) :
・If the rib is thick, the tip of the flow which has dried up flows diagonally into the rib, and slides along the base of the rib. When the dried material gets caught at the exit of the rib, it temporarily
・If the rib is thin, the tip of the flow enters the rib and reaches the base of the rib. After that, the material that follows generally does not enter the rib, and bypasses thebaseofthe rib.
(2) Flow behavior of area whose thickness can be decreased (groove or hole) :
・With very thin walls of under０.２mm（groove）, the material is unable to flow into the thin walls and detours to both sides of the thin-walled groove.
・In the case of obstacle plates（hole）, as the material slides jetting from both sides of the obsta-cle plate, a large space is formed at the back of the plate. Subsequently this causes the material to fill into the space from both sides. When the space is finally filled and disappears, shear flow lines are generated along both sides of a stagnant triangular area at the back of the plate.

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