A honeycomb filter includes cell walls and cells. The cell walls extend along a longitudinal direction of the honeycomb filter to define the cells between the cell walls. The cells have first cells and second cells each having a substantially square shape in a cross-section perpendicular to the longitudinal direction. The first cells are sealed and the second cells are open at one end face of the honeycomb filter. The substantially square shape of the first cells includes a first side whose extended line intersects a second side except for corner portions of the substantially square shape of the second cells. The second side and the extended line of the first side are substantially perpendicular.