An optimum adhesive penetration is needed to provide satisfactory bonding strength at the veneer–veneer interface. The effect of veneer density and glue spreading rate on the phenol formaldehyde adhesive penetration plot profile was determined. An X-ray apparatus was used to visualize the adhesive penetration of the plywood. The heartwood and sapwood veneer of Cryptomeria japonica with low, medium, and high veneer densities were made into plywood. The glue spreading rate was applied from 75 g/m2 to 225 g/m2 for the heartwood plywood and up to 300 g/m2 for the sapwood plywood (plus 75 g/m2 at every level of glue spreading rate). An X-ray apparatus with a low tube voltage successfully visualized the adhesive penetration plot profile. Based on the half-width calculation, the adhesive penetration depth ranged from 0,3 – 0,9 mm. The mean half-width was 0,5 mm. The adhesive concentration increased with increasing glue spreading rate. In contrast, it also showed that using different veneer densities and increasing glue spreading rates does not affect the half-width value as the adhesive penetrates deeper.