To provide a partially diluting device for accurately and precisely measuring the amount of a particulate substance contained in the exhaust gas of an engine.

The partially diluting tunnel device extracts one portion of the exhaust gas Ge in the engine for guiding to a diluting tunnel body 2, supplies a diluted exhaust gas Gm generated by diluting the exhaust gas Ge at the dilution tunnel body 2 by using diluting air A to a filter holder 9, and captures the particulate substance in the diluted exhaust gas Gm. The partially dilution tunnel device is so constituted that, a flow rate F1 in the diluted air A can be controlled to a nearly prescribed flow rate, a flow rate F2 of the diluted exhaust gas Gm can be controlled to nearly a prescribed flow rate, and the diluted exhaust gas Gm in the flow rate F3 obtained by multiplying a flow rate Qe in the exhaust gas Ge of the engine by a specified ratio α can be supplied to a filter holder 9.