An air flow measuring device for measuring air flow flowing through an air intake passage of an internal combustion engine by detecting air flow flowing through a sub-passage disposed in the air intake passage, the air flow flowing through the sub-passage being a part of the air flow flowing through the air intake passage, said air flow measuring device comprising: a flow measuring element to measure air flow; and a housing wherein the sub-passage is formed therein and the flow measuring element is disposed in the sub-passage, said housing including one end portion with a flange for installing the air flow measuring device in the air intake passage, an upstream side and a downstream side opposite to the upstream side; wherein the sub-passage includes: an inlet portion provided on said upstream side of said housing and having an opening face which is perpendicular to the air flow in the intake passage; an outlet portion; a first vertical path whose one end communicates with the inlet portion and another end communicates with a vent provided at the downstream end of an inclination, a part of the air entering the inlet portion flows on the first vertical path straight down toward the inclination and the vent; and a branching path (74, 75) which branches off from the first vertical path and communicates with the outlet portion, wherein the flow measuring element is disposed at an intermediate area in the branching path (74, 75), characterized in that said branching path branches off at a portion upstream of the inclination and in that the branching path includes a first horizontal path branching off from the first vertical path between the inlet portion and the vent toward said one end portion of the housing with the flange and a second vertical path communicating with the first horizontal path at a downstream end of the first horizontal path so as to communicate the first horizontal path with the outlet portion, which is on the downstream side of said housing, wherein the second vertical path is arranged in a direction from the upstream side of the housing to the downstream side of the housing of the air flow flowing through the air intake passage; and wherein the outlet portion has an opening face parallel to the air flow in the air intake passage. a flow measuring element to measure air flow; and a housing wherein the sub-passage is formed therein and the flow measuring element is disposed in the sub-passage, said housing including one end portion with a flange for installing the air flow measuring device in the air intake passage, an upstream side and a downstream side opposite to the upstream side; wherein the sub-passage includes: an inlet portion provided on said upstream side of said housing and having an opening face which is perpendicular to the air flow in the intake passage; an outlet portion; a first vertical path whose one end communicates with the inlet portion and another end communicates with a vent provided at the downstream end of an inclination, a part of the air entering the inlet portion flows on the first vertical path straight down toward the inclination and the vent; and a branching path (74, 75) which branches off from the first vertical path and communicates with the outlet portion, an inlet portion provided on said upstream side of said housing and having an opening face which is perpendicular to the air flow in the intake passage; an outlet portion; a first vertical path whose one end communicates with the inlet portion and another end communicates with a vent provided at the downstream end of an inclination, a part of the air entering the inlet portion flows on the first vertical path straight down toward the inclination and the vent; and a branching path (74, 75) which branches off from the first vertical path and communicates with the outlet portion, wherein the flow measuring element is disposed at an intermediate area in the branching path (74, 75), characterized in that said branching path branches off at a portion upstream of the inclination and in that the branching path includes a first horizontal path branching off from the first vertical path between the inlet portion and the vent toward said one end portion of the housing with the flange and a second vertical path communicating with the first horizontal path at a downstream end of the first horizontal path so as to communicate the first horizontal path with the outlet portion, which is on the downstream side of said housing, wherein the second vertical path is arranged in a direction from the upstream side of the housing to the downstream side of the housing of the air flow flowing through the air intake passage; and wherein the outlet portion has an opening face parallel to the air flow in the air intake passage.