A PM amount detecting apparatus having a PM sensor installed in a sensor case into which a part of exhaust gas of an internal combustion engine allowed to flow through an exhaust gas passage is intaken. The sensor case has a structure which lowers a flow rate of the exhaust gas therein to such an extent that PM is capable of performing thermal phoresis, and a structure which generates therein such a temperature difference that PM is guided to the PM sensor in accordance with the thermal phoresis.