A fault diagnosis apparatus for an airflow meter for detecting an actual intake air quantity, comprising: means for detecting a rotation speed of an internal combustion engine; and means for estimating an intake air quantity, wherein it is determined that the airflow meter has a fault when a divergence value (AFMDG) of the estimated intake air quantity (Qest) calculated by the intake-air-quantity estimation means with respect to the actual intake air quantity obtained by the airflow meter is kept within a fault determination area set outside of a specified area (THlow≤AFMDG≤THhigh) sandwiched between an upper-limit fault diagnosis criterion (THhigh) and a lower-limit fault diagnosis criterion (THlow), both determined based on the rotation speed of the internal combustion engine, wherein the estimated intake air quantity (Qest) is calculated by correcting a base estimated intake air quantity (Qesb) based on both the rotation speed and a degree of throttle opening of the internal combustion engine by a quantity of valve overlap between an intake valve and an exhaust valve as an operation control parameter of the internal combustion engine except both the rotation speed and the degree of throttle opening of the internal combustion engine, wherein the upper-limit fault diagnosis criterion (THhigh) and the lower-limit fault diagnosis criterion (THlow) for determining that the airflow meter has a fault is set to narrow the fault determination area, as the rotation speed of the internal combustion engine reduces, and wherein the upper-limit fault diagnosis criterion (THhigh) increases and the lower-limit fault diagnosis criterion (THlow) decreases, as the rotation speed of the internal combustion engine reduces, thereby permitting the fault determination area to be narrowed. means for detecting a rotation speed of an internal combustion engine; and means for estimating an intake air quantity, wherein it is determined that the airflow meter has a fault when a divergence value (AFMDG) of the estimated intake air quantity (Qest) calculated by the intake-air-quantity estimation means with respect to the actual intake air quantity obtained by the airflow meter is kept within a fault determination area set outside of a specified area (THlow≤AFMDG≤THhigh) sandwiched between an upper-limit fault diagnosis criterion (THhigh) and a lower-limit fault diagnosis criterion (THlow), both determined based on the rotation speed of the internal combustion engine, wherein the estimated intake air quantity (Qest) is calculated by correcting a base estimated intake air quantity (Qesb) based on both the rotation speed and a degree of throttle opening of the internal combustion engine by a quantity of valve overlap between an intake valve and an exhaust valve as an operation control parameter of the internal combustion engine except both the rotation speed and the degree of throttle opening of the internal combustion engine, wherein the upper-limit fault diagnosis criterion (THhigh) and the lower-limit fault diagnosis criterion (THlow) for determining that the airflow meter has a fault is set to narrow the fault determination area, as the rotation speed of the internal combustion engine reduces, and wherein the upper-limit fault diagnosis criterion (THhigh) increases and the lower-limit fault diagnosis criterion (THlow) decreases, as the rotation speed of the internal combustion engine reduces, thereby permitting the fault determination area to be narrowed.