A bone treating device has a bone plate and a bone screw. The bone plate has an opening which penetrates therethrough and has a female screw part having a plurality of thread grooves formed on an inner surface of the opening. The bone screw has a shaft part capable of penetrating the opening of the bone plate and a head part provided at a proximal end of the shaft part. The head part has on an outer surface thereof a male screw part having a plurality of screw threads threadedly engageable with the thread grooves of the female screw part formed on the inner surface of the opening. The bone screw has a threaded engagement start guiding portion formed by extending screw threads whose number is fewer by at least one than the number of screw threads formed in the male screw part to a threaded engagement start side.