An apparatus for designing a semiconductor integrated circuit according to an embodiment of the present invention includes an interface circuit information extracting unit configured to specify one or more transmitting registers and one or more receiving registers forming an interface that needs to be synchronized, an insertion point candidate specifying unit configured to specify a number of supply sources that is of a number of the transmitting registers serving as data supply sources, for each receiving register, and specify at least one insertion point candidate based on the number of supply sources, an insertion point specifying unit configured to specify a number of output destinations that is of a number of the receiving registers serving as data output destinations, for each transmitting register, and specify at least one insertion point based on the number of output destinations and the insertion point candidate, and a synchronization circuit inserting unit configured to insert a synchronization circuit in the insertion point, and generate synchronized circuit description data of the semiconductor integrated circuit in which the synchronization circuit is inserted in the insertion point.