Disclosed is a method of preparing isobutene in which high-purity isobutene is separated (prepared) from a C4 mixture by cracking glycol ether prepared from a C4 mixture (in particular, C4 raffinate-1) containing isobutene and a glycol. The method includes cracking glycol ether into isobutene and glycol at a temperature between 50° C. and 300° C. in the presence of a strongly acidic catalyst. The glycol ether may be prepared by reaction between a C4 mixture containing isobutene and glycol in the presence of an acid catalyst.