A filler for substrates which comprises an inorganic substance and a carbodiimide-group-containing organic layer chemically bonded to the surface of the inorganic substance. Examples of the carbodiimide-group-containing organic layer include a layer comprising a carbodiimide-group-containing compound represented by either of the following formulae. Due to the constitution, the filler for substrates can be highly dispersed in a resin for substrates even when added in a high proportion and can give a substrate inhibited from deteriorating in electrical properties, mechanical properties, etc. X 1 m - Z - A - R 1 - N = C = N n - R 1 - NCO 1 X 1 m - Z - A - R 1 - N = C = N n - R 1 - A - Z - X 2 3 1
[R 1 represents a residue of an isocyanate compound; X 1 and X 2 each independently represents hydrogen, halogeno, etc.; Z's each independently represents silicon or titanium; A represents an organic group having a valence of 2 or higher containing a bond derived from an isocyanate group; m and 1 each is an integer of 1-3, provided that m+1=4; and n is an integer of 1-100.]