Plastic materials are widely used for a variety of purposes due to their useful properties, but there are problems such as the environmental burden associated with the use of fossil fuels and the depletion of resources. Under such circumstances, WPCs (Wood Plastic Composites), which are composites of resin and wood flour, have been attracting attention as a material that can reduce the amount of plastic derived from fossil resources. In this study, we focused on the WPCs and researched the flame retardant effect obtained by adding cellulose. In the previous research, it demonstrated that polypropylene (PP) with powdered cellulose (CP) and ammonium polyphosphate (APP) could achieve efficient flame retardance by the formation of a solid phase called the char layer. In this study, the formation of char layer, which is a necessary flame retardant factor to achieve UL94 V-0 rating, was investigated for PP/CP/APP and PP/wood flour (WF)/APP composites. In our study, a flame retardant criterion was established based on the total heat release during combustion by cone calorimeter test, and the relationships between the criterion, the char layer formation rate and the char layer density was investigated. The results indicated that it is not the char layer formation rate but the char layer density that is necessary to achieve flame retardance of the material.