A control unit of an internal combustion engine comprising: an exhaust gas recirculation control part for sending an EGR command signal so as to bring an EGR gas supply amount by an EGR apparatus to a target EGR gas supply amount to control an opening degree of an EGR valve; a supercharger control part for sending a supercharging command signal so as to bring a supercharging amount of a supercharger provided with a variable flow mechanism to a target supercharging amount to control an opening degree of the variable flow mechanism; an in-cylinder oxygen excess ratio calculation part for calculating an in-cylinder oxygen excess ratio based on an operating state of the internal combustion engine; a transient state determination part for determining whether the internal combustion engine is in a transient state based on the oxygen excess ratio in the cylinder calculated by the in-cylinder oxygen excess ratio calculation part; and a switching part for switching the EGR command signal and the supercharging command signal so that the opening degree of the EGR valve and the opening degree of the variable flow mechanism become smaller than in a normal state when the internal combustion engine is determined to be in the transient state by the transient state determination part; the control unit being characterised in that the transient state determination part comprises: a first determination part for determining whether the in-cylinder oxygen excess ratio calculated by the in-cylinder oxygen excess ratio calculation part is not greater than a first threshold value; and a second determination part for determining whether an in-cylinder oxygen excess ratio difference between the in-cylinder oxygen excess ratio calculated by the in-cylinder oxygen excess ratio calculation part and an optimum value of the in-cylinder oxygen excess ratio which is preliminarily set on the basis of the operating state of the internal combustion engine is not less than a second threshold value; and is configured to determine the internal combustion engine to be in the transient state when the first determination part determines that the in-cylinder oxygen excess ratio is not greater than the first threshold value and the second determination part determines that the in-cylinder oxygen excess ratio difference is not less than the second threshold value. an exhaust gas recirculation control part for sending an EGR command signal so as to bring an EGR gas supply amount by an EGR apparatus to a target EGR gas supply amount to control an opening degree of an EGR valve; a supercharger control part for sending a supercharging command signal so as to bring a supercharging amount of a supercharger provided with a variable flow mechanism to a target supercharging amount to control an opening degree of the variable flow mechanism; an in-cylinder oxygen excess ratio calculation part for calculating an in-cylinder oxygen excess ratio based on an operating state of the internal combustion engine; a transient state determination part for determining whether the internal combustion engine is in a transient state based on the oxygen excess ratio in the cylinder calculated by the in-cylinder oxygen excess ratio calculation part; and a switching part for switching the EGR command signal and the supercharging command signal so that the opening degree of the EGR valve and the opening degree of the variable flow mechanism become smaller than in a normal state when the internal combustion engine is determined to be in the transient state by the transient state determination part; the control unit being characterised in that the transient state determination part comprises: a first determination part for determining whether the in-cylinder oxygen excess ratio calculated by the in-cylinder oxygen excess ratio calculation part is not greater than a first threshold value; and a second determination part for determining whether an in-cylinder oxygen excess ratio difference between the in-cylinder oxygen excess ratio calculated by the in-cylinder oxygen excess ratio calculation part and an optimum value of the in-cylinder oxygen excess ratio which is preliminarily set on the basis of the operating state of the internal combustion engine is not less than a second threshold value; and is configured to determine the internal combustion engine to be in the transient state when the first determination part determines that the in-cylinder oxygen excess ratio is not greater than the first threshold value and the second determination part determines that the in-cylinder oxygen excess ratio difference is not less than the second threshold value. a first determination part for determining whether the in-cylinder oxygen excess ratio calculated by the in-cylinder oxygen excess ratio calculation part is not greater than a first threshold value; and a second determination part for determining whether an in-cylinder oxygen excess ratio difference between the in-cylinder oxygen excess ratio calculated by the in-cylinder oxygen excess ratio calculation part and an optimum value of the in-cylinder oxygen excess ratio which is preliminarily set on the basis of the operating state of the internal combustion engine is not less than a second threshold value; and is configured to determine the internal combustion engine to be in the transient state when the first determination part determines that the in-cylinder oxygen excess ratio is not greater than the first threshold value and the second determination part determines that the in-cylinder oxygen excess ratio difference is not less than the second threshold value.