1. A control apparatus comprising: a microcomputer configured to: learn a control parameter in association with a plurality of variables; compute the control parameter corresponding to current variables which represent a current environmental condition by interpolating the learned control parameter; and control a control object based on the control parameter computed by the interpolation, wherein a learning is executed by correcting a learning vector comprising the variables and the control parameter based on a measurement vector comprising measured values of the variables and a measured value of the control parameter, three learning vectors are selected from a plurality of learned learning vectors, the control parameter corresponding to the current variables is computed by interpolating the control parameters on a flat surface including said three learning vectors, and the three learning vectors are selected in such manner that the current variables are positioned inside of a triangle connecting said three learning vectors.