A transmission assembly structure includes an engine-side joining surface, a transmission-side joining surface, which is joined to the engine-side joining surface, an engine component which is provided on an outside of the engine-side joining surface in a radial direction in the flywheel housing and protrudes toward the transmission side, a guide member which is provided on the engine-side joining surface or in the vicinity of the engine-side joining surface of the flywheel housing and in a vicinity of the engine component and which protrudes toward the transmission side further than the engine component, and a guide hole, which is provided on the transmission-side joining surface of the transmission housing or in the vicinity of the transmission-side joining surface and into which the guide member is inserted.