In terms of the problem of formaldehyde-releasing, glyoxal is one of the interesting dialdehydes as a crosslinking agent for wood. In the present report, the reaction of glyoxal and glyoxal/glycol with the wood of Japanese ash (Fraxinus mandshurica Rupr. var. japonica Maxim.) has been studied. Particular attention to the structure of the product (after treatment with glyoxal/glycol (glycol: ethylene glycol, diethylene glycol, triethylene glycol)) was made and the formation of crosslinking has been discussed. Reaction of only glyoxal with wood showed little reactivity, while that of glyoxal/glycol gave high weight increases for each glycol. The appropriate condition was at 100° to 130°C for 2 hrs. It was found that the product forms network structures, at least within itself. The crosslinking between the product and wood components, however, could not be confirmed, although it was assumed that the product was structurally linked to wood by its reactive sites.