Patent ID: 7326270

Claim:
A filter comprising: a honeycomb structural body having a plurality of through holes that are placed in parallel with one another in a length direction with wall portion interposed therebetween, wherein: said through holes comprises two types of through holes, which are large-capacity through holes having a relatively larger cross-sectional area in a cross section perpendicular to the length direction, and small-capacity through holes having a relatively smaller cross-sectional area in said cross section, with the numbers of the two types of through holes being substantially same; said large-capacity through holes are sealed at one end of said filter, while said small-capacity through holes are sealed at the other end of said filter; and supposing that a ratio (a/b) of a total (a) of lengths in said cross section of wall surfaces of one large-capacity through hole, the wall surfaces being shared by said one large-capacity through hole and adjacent large-capacity through holes in the cross section to a total (b) of lengths in said cross section of wall surfaces of one large-capacity through hole, the wall surfaces being shared by said one large-capacity through hole and adjacent small-capacity through holes in the cross section is defined as α and that a ratio (A/B) of an area (A) in said cross section of said large-capacity through hole to an area (B) in said cross section of said small-capacity through hole is defined as β, α and β satisfy the relationship represented by the following formulas 0<α≦1.5, 2 ≦β≦2.75, and β≧(20/9)α 2 +1.