Abstract: The invention provides an internal combustion engine controller for an internal combustion engine having a fuel tank which can be purged of fuel vapor via a purge line connected through a canister unit, to a suction input of the internal combustion engine. The amount of fuel vapor stored in the canister or the amount of fuel vapor supplied to the internal combustion engine according to the operating state and operation range of the engine are calculated, and the purge control valve and air-fuel ratio control characteristics are adjusted using the calculated value. At the same time, appropriate control of the amount of fuel vapor and purge control valve when diagnosing the purge pipe system failure by application of pressure to the purge pipe system, are provided. Both internal combustion engine control with the minimum change in air-fuel ratio and purge control are ensured, and highly reliable diagnosis of the purge pipe system is realized.