A control method for an internal combustion engine, the internal combustion engine including: - a variable compression ratio mechanism which is capable of modifying a mechanical compression ratio of the internal combustion engine; - a supercharger which is configured in order to supercharge intake air; and - a supercharging pressure adjustment mechanism which is configured in order to adjust a supercharging pressure, the internal combustion engine carrying out a fuel increment to a richer side than a stoichiometric air-fuel mixture ratio at a high supercharging pressure side, the control method comprising: - detecting the mechanical compression ratio from a device for detecting an actual compression ratio modified by the variable compression ratio mechanism; - setting an upper limit value of the supercharging pressure on a basis of the mechanical compression ratio; and - adjusting (S7, S8, S9, S10) the supercharging pressure not to exceed the upper limit value of the supercharging pressure, the upper limit value of the supercharging pressure being within a region in which the fuel increment is carried out, wherein: - as the mechanical compression ratio becomes higher, the upper limit value of the supercharging pressure is made lower, - in a transient state in which a demand torque is raised, the control method determining a degree of pace of a rise in the demand load and adjusting the supercharging pressure not to exceed the upper limit value of the supercharging pressure in a case where the degree of pace of the rise in the demand load is abrupt, and - in a case where the degree of pace of the rise in the demand load is moderate, the supercharging pressure is adjusted not to exceed a second upper limit value of the supercharging pressure which is lower than the upper limit value of the supercharging pressure and which falls in a region in which a driving at the stoichiometric air-fuel mixture ratio is possible. - a variable compression ratio mechanism which is capable of modifying a mechanical compression ratio of the internal combustion engine; - a supercharger which is configured in order to supercharge intake air; and - a supercharging pressure adjustment mechanism which is configured in order to adjust a supercharging pressure, - detecting the mechanical compression ratio from a device for detecting an actual compression ratio modified by the variable compression ratio mechanism; - setting an upper limit value of the supercharging pressure on a basis of the mechanical compression ratio; and - adjusting (S7, S8, S9, S10) the supercharging pressure not to exceed the upper limit value of the supercharging pressure, the upper limit value of the supercharging pressure being within a region in which the fuel increment is carried out, - as the mechanical compression ratio becomes higher, the upper limit value of the supercharging pressure is made lower, - in a transient state in which a demand torque is raised, the control method determining a degree of pace of a rise in the demand load and adjusting the supercharging pressure not to exceed the upper limit value of the supercharging pressure in a case where the degree of pace of the rise in the demand load is abrupt, and - in a case where the degree of pace of the rise in the demand load is moderate, the supercharging pressure is adjusted not to exceed a second upper limit value of the supercharging pressure which is lower than the upper limit value of the supercharging pressure and which falls in a region in which a driving at the stoichiometric air-fuel mixture ratio is possible.