A diagnosis device for an internal combustion engine that is mounted on a vehicle, the internal combustion engine including an EGR device and an air flow meter configured to detect an intake air amount, the diagnosis device comprising: an electronic control unit configured to: execute EGR diagnosis processing to diagnose whether or not the EGR device operates normally while the vehicle is decelerated; execute air flow meter diagnosis processing to diagnose whether or not the air flow meter is normal while the vehicle is decelerated; diagnose the EGR device as normal in the EGR diagnosis processing when a differential pressure between a first intake pressure and a second intake pressure is equal to or higher than an abnormality determination value, the first intake pressure being an intake pressure in an intake pipe of the internal combustion engine that is determined when an EGR valve of the EGR device is closed, based on the intake air amount detected by the air flow meter, and the second intake pressure being an intake pressure in the intake pipe that is determined when the EGR valve is open, based on the intake air amount detected by the air flow meter; and either: execute the EGR diagnosis processing after the air flow meter diagnosis processing has been completed; or execute the EGR diagnosis processing when the air flow meter diagnosis processing has not been completed and a predetermined condition that a duration of deceleration of the vehicle is shorter than a diagnosis time that is required to execute the air flow meter diagnosis processing are fulfilled, wherein the predetermined condition includes a condition that a vehicle speed that is determined when the vehicle starts to be decelerated is lower than a determination value. execute EGR diagnosis processing to diagnose whether or not the EGR device operates normally while the vehicle is decelerated; execute air flow meter diagnosis processing to diagnose whether or not the air flow meter is normal while the vehicle is decelerated; diagnose the EGR device as normal in the EGR diagnosis processing when a differential pressure between a first intake pressure and a second intake pressure is equal to or higher than an abnormality determination value, the first intake pressure being an intake pressure in an intake pipe of the internal combustion engine that is determined when an EGR valve of the EGR device is closed, based on the intake air amount detected by the air flow meter, and the second intake pressure being an intake pressure in the intake pipe that is determined when the EGR valve is open, based on the intake air amount detected by the air flow meter; and either: execute the EGR diagnosis processing after the air flow meter diagnosis processing has been completed; or execute the EGR diagnosis processing when the air flow meter diagnosis processing has not been completed and a predetermined condition that a duration of deceleration of the vehicle is shorter than a diagnosis time that is required to execute the air flow meter diagnosis processing are fulfilled, wherein the predetermined condition includes a condition that a vehicle speed that is determined when the vehicle starts to be decelerated is lower than a determination value. execute the EGR diagnosis processing after the air flow meter diagnosis processing has been completed; or execute the EGR diagnosis processing when the air flow meter diagnosis processing has not been completed and a predetermined condition that a duration of deceleration of the vehicle is shorter than a diagnosis time that is required to execute the air flow meter diagnosis processing are fulfilled, wherein the predetermined condition includes a condition that a vehicle speed that is determined when the vehicle starts to be decelerated is lower than a determination value.