A transmission apparatus has a transmission casing which is divided into two parts, namely, a front part casing, shaped generally as a frustum of a cone for forming a front part of the transmission casing and a rear part casing shaped generally as a frustum of a cone, for forming a rear part of the transmission casing. The rear part casing and the front part casing are connected to each other so as to form the transmission casing which has a cross section which gradually decreases from the front to the back of the transmission casing.
