To provide an air-fuel ratio control device and an engine control system maintaining good drivability, while alternately repeating an air-fuel ratio on a rich side and an air-fuel ratio on a lean side.

The air-fuel ratio control device (ECU for engine control) controls the air-fuel ratio as a ratio between an air amount and a fuel amount according to fuel combustion in an engine. The air-fuel ratio control device comprises a program variably controlling an average air-fuel ratio of a predetermined period (a cycle of each pattern) including a first period and a second period, during warm-up at the initial stage of engine start, while alternately repeating the first period (period of R) and the second period (period of L) in Fig. 10 (b)-(d), continuously or through a reference air-fuel ratio (S). The first period uses the air-fuel ratio richer than the predetermined reference air-fuel ratio (stoichiometric air-fuel ratio) as a control target value in the first period, and the second period uses the air-fuel ratio leaner than the predetermined reference air-fuel ratio as a control target value in the second period.