A drum type washing machine, comprising: a tub having an air hole which is formed at an upper portion of a rear wall of the tub, permitting fluid communication between an inside of the tub and an outside of the tub, the tub configured to hold water; a drum inside the tub, the drum configured to accommodate laundry and rotate; a water pressure and/or level sensor configured to sense a level and/or pressure of water in the tub; an air flow path connected with the air hole of the tub and configured to permit fluid communication between the inside of the tub and the outside of the tub; a bubble detecting sensor along the air flow path, the bubble detecting sensor configured to detect bubbles, suds or foam, wherein the bubble detecting sensor includes a bubble sensing chamber along the air flow path, and the bubble sensing chamber having a larger cross-sectional area than a cross-sectional area of the air flow path; and a controller configured to perform a bubble removing procedure according to an indication of the bubble detecting sensor characterized in that the bubble detecting sensor is installed in or along the air flow path, wherein the bubble sensing chamber is connected between a first hose and a second hose, wherein the first hose is connected to the air hole of the tub and the second hose branches into a third hose and a fourth hose, wherein the third hose guides water discharged through the first hose along a discharge flow path, wherein the fourth hose is directed toward the upper portion of the rear wall of the tub to enable gaseous and/or fluid communication between the inside of the tub and the outside of the tub. a tub having an air hole which is formed at an upper portion of a rear wall of the tub, permitting fluid communication between an inside of the tub and an outside of the tub, the tub configured to hold water; a drum inside the tub, the drum configured to accommodate laundry and rotate; a water pressure and/or level sensor configured to sense a level and/or pressure of water in the tub; an air flow path connected with the air hole of the tub and configured to permit fluid communication between the inside of the tub and the outside of the tub; a bubble detecting sensor along the air flow path, the bubble detecting sensor configured to detect bubbles, suds or foam, wherein the bubble detecting sensor includes a bubble sensing chamber along the air flow path, and the bubble sensing chamber having a larger cross-sectional area than a cross-sectional area of the air flow path; and a controller configured to perform a bubble removing procedure according to an indication of the bubble detecting sensor characterized in that the bubble detecting sensor is installed in or along the air flow path, wherein the bubble sensing chamber is connected between a first hose and a second hose, wherein the first hose is connected to the air hole of the tub and the second hose branches into a third hose and a fourth hose, wherein the third hose guides water discharged through the first hose along a discharge flow path, wherein the fourth hose is directed toward the upper portion of the rear wall of the tub to enable gaseous and/or fluid communication between the inside of the tub and the outside of the tub.