Letting the maximum distance of penetration of {TAIL ~ exhaust gas} flowing through the cells 22 into {HEAD ~ the cell wall} 23 be “d,” the exhaust gas flowing through each cell 22 is believed to be able to reach substantially all the region of the cell wall 23 in the honeycomb unit 130 where the corner parts of the cells 22 are not provided with a curved surface as shown in FIG 1 if the cell wall 23 has a thickness “x” less than or equal to (d·√{square root over }) 
Letting the maximum distance of penetration of { TAIL ~ exhaust gas } flowing through the cells 22 into { HEAD ~ the cell wall } 23 be “ d , ” the exhaust gas flowing through each cell 22 is believed to be able to reach substantially all the region of the cell wall 23 in the honeycomb unit 130 where the corner parts of the cells 22 are not provided with a curved surface as shown in FIG 1 if the cell wall 23 has a thickness “ x ” less than or equal to ( d·√{square root over } )
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