To provide an injecting device of hydrophobic material for a culvert, capable of efficiently injecting hydrophobic material for the culvert.

A device (10) comprises a lower portion (12) having a couple of side walls which are disposed facing each other across a width approximately equal to the width of a ditch to be dug by a trencher for burying a culvert drain pipe and a front wall which connects to each of the front ends of the side walls, an upper portion (14) which connects to the top of the lower portion and has a hopper shape, and a hydrophobic material adjustment plate (16) which is arranged to close the rear portion of the lower portion and allows for vertical position adjustment. With the trencher traveling, the hydrophobic material is injected from the upper portion into the ditch through the lower portion in a state that the lower portion is located in the ditch. Consequently, the lower end (16b) of the hydrophobic material adjustment plate levels the hydrophobic material so as to maintain the predetermined height H of the top face of the hydrophobic material from the ground level.