A coating agent has been applied to microneedles having a biodegradable resin, and a relation between its strength and performance has been clarified. As a result thereof, it has been found that the application of the coating agent to microneedles of a biodegradable resin having a predetermined strength or more, that is, a predetermined weight average molecular weight, allows the microneedles to have excellent performance. Further, it has been found that in regard to the coating agent, a predetermined type and amount thereof are also useful to demonstrate the performance. In view of this, when a microneedle device 1 is arranged so that at least a part of a microneedle array 2 provided with microneedles 3 made from polylactic acid having a weight average molecular weight of 40,000 or more is coated with a carrier containing physiologically active ingredients, it is possible to attain excellent migration characteristics of the physiologically active ingredients to a living body.