A filter for the filtration of molten aluminum is obtained by binding 100 parts by weight of aggregate particles with 10 to 20 parts by weight of an inorganic binding material, said aggregate particles being composed of at least one alumina selected from the group consisting of fused alumina and sintered alumina and said inorganic binding material having a raw material composition comprising 25 wt. % to 35 wt. % of SiO.sub.2, 30 to 40 wt. % of B.sub.2 O.sub.3, 20 to 35 wt. % of Al.sub.2 O.sub.3 and the balance being MgO. A filter according to the invention has a crystallinity of from 10% to 25% with the proviso that in the powder X-ray diffraction, the crystallinity is expressed in terms of a percentage of a peak height of 9Al.sub.2 O.sub.3 .multidot.2B.sub.2 O.sub.3 appearing at 2.theta.=16.5.degree. to a peak height of .alpha.-Al.sub.2 O.sub.3 appearing at 2.theta.=43.4.degree.. Needle crystals of 9Al.sub.2 O.sub.3 .multidot.2B.sub.2 O.sub.3 which are formed in the inorganic binding material are not longer than 10 .mu.m. Needle crystals of 9Al.sub.2 O.sub.3 .multidot.2B.sub.2 O.sub.3 formed in the inorganic binding material can be obtained by firing the filter at 1,200.degree. C. to 1,300.degree. C.