PURPOSE: To make the gear noise of a transmission body directly measurable in making its internal sound pressure be hard to receive any effect of other equipment by measuring a degree of sound pressure in the case of a transmission by a sound pressure measuring device installed in the case.

CONSTITUTION: Each rotation of an electric motor and a turbine or the like is transmitted to an input shaft 1 via a coupling and so on, and this rotation is transmitted to a gear 4 from a pinion 3, rotating an output shaft 2. With this, rotation of the input shaft 1 is decelerated, changing its direction, and transmitted to the output shaft 2. Likewise, rotation of the output shaft 2 is transmitted to the load side via the coupling or the like, and load of a pump or the like is driven, by way of example. At this time, impact and vibration noises are produced by engagement between the pinion 3 and the gear 4. In this transmission, since a sound pressure measuring device 13 measuring the extent of sound pressure in a case is installed in this case 5, this sound pressure in the case is directly measurable, Accordingly, in the noise in the case, a main component of the sound pressure is of mesh frequency component, so any noise due to gear engagement is well manifestable.