I referred to this entry. (http://d.hatena.ne.jp/griefworker/20111117/backbone_sync)

backbone-cacheSync.js

var collection = Backbone.Collection.extend({

    originalSync: Backbone.sync,

    sync: function(method, model, options) {

        var originalSuccess, cache;

        if (method !== 'read') {
            this.originalSync(method, model, options);
            return;
        }

        originalSuccess = options.success;
        cache = localStrage.getItem(model.url);

        if (cache) {
            originalSuccess(JSON.parse(cache));
            return;
        }

        options.success = function(collection) {
            localStorage.setItem(model.url, JSON.stringify(collection));
            originalSuccess(collection);
        };

        this.originalSync(method, model, options);
    }
});