1. A control apparatus comprising: a learning portion which learns a control parameter in association with a plurality of variables; an interpolation portion which computes the control parameter corresponding to current variables which represent a current environmental condition by interpolating the control parameter learned by the learning portion; and a control portion which controls a control object based on the control parameter computed by the interpolation portion, wherein the learning portion executes a learning by correcting the learning vector consisting of the variables and the control parameter based on a measurement vector consisting of measured values of the variables and a measured value of the control parameter, and the interpolating portion includes a selecting portion which selects three learning vectors from a plurality of learning vectors learned by the learning portion, and the interpolating portion computes the control parameter corresponding to the current variables by interpolating the control parameters on a flat surface including said three learning vectors.