Given an undirected graph on a node set V and positive integers k and m, a k-connected m-dominating set ((k, m)-CDS) is defined as a subset S of V such that each node in V \ S has at least m neighbors in S, and a k-connected subgraph is induced by S. The weighted (k, m)-CDS problem is to find a minimum weight (k, m)-CDS in a given node-weighted graph. The problem is called the unweighted (k, m)-CDS problem if the objective is to minimize the cardinality of a (k, m)-CDS. These problems have been actively studied for unit disk graphs, motivated by the application of constructing a virtual backbone in a wireless ad hoc network. In this paper, we consider the case in which k ≤ m, and we present a simple O(k5 k )-approximation algorithm for the unweighted (k, m)-CDS problem, and a primal-dual O(k 2 log k)-approximation algorithm for the weighted (k, m)-CDS problem.A CDS does not give a fault-tolerant virtual backbone network. This is because a CDS is only required to be connected, and each node outside a CDS is required to have only one neighbor in the CDS. Hence, if a backbone node fails, the virtual backbone network may be disconnected, or a nonbackbone node may lose access to the virtual backbone network. To overcome this disadvantage, Dai and Wu [10] proposed replacing a CDS by a k-connected k-dominating set, and they addressed the problem of finding a minimum k-connected k-dominating set in a unit disk graph. For a graph with the node set V , a subset S of V is called k-connected if the subgraph induced by S is kconnected (i.e., it is connected even if any k − 1 nodes are removed), and is called k-dominating if each node v ∈ V \ S has k neighbors in S. Triggered by their study, much attention has been paid to this problem, extending the notion of a k-connected k-dominating set to a more-general k-connected m-dominating set ((k, m)-CDS).The problem of finding a minimum cardinality (k, m)-CDS in a unit disk graph is called the unweighted (k, m)-CDS problem. If each node is given a nonnegative weight, and the objective is to minimize the weight of a (k, m)-CDS, then this is called the weighted (k, m)-CDS problem. As for the unweighted (k, m)-CDS problem, several constant-approximation algorithms were given for k ≤ 3 [21,22,26,27,30]. As for the weighted (k, m)-CDS problem, there are several constantapproximation algorithms for k = m = 1 [2, 31], but no approximation algorithm was known for the case of (k, m) = (1, 1) before our study (see Section 2 for more literature reviews).After these previous studies, a natural question arises as to whether there is a constantapproximation algorithm for the unweighted (k, m)-CDS problem with k ≥ 4, and for the weighted problem with (k, m) = (1, 1). For the unweighted problem, this question has been already addressed in both [26] and [27].