This paper addresses how to design a cache algorithm for achieving high cache hit rate and high packet forwarding rate of Named Data Networking (NDN) software routers. Although sophisticated cache eviction algorithms like LFU and Adaptive Replacement Cache (ARC) successfully achieve high cache hit rate, they incur heavy computational overheads to choose a victim Data packet from the cache. In contrast, cache admission is expected to achieve high cache hit rate with light-weight computation since it decides simply whether an incoming Data packet should be inserted into the cache or not. In this paper, we design a frequency-based cache admission algorithm, Filter, with light-weight computation by simply counting frequencies of incoming Data packets in a fixed time window. A simulation-based evaluation proves that Filter achieves high cache hit rate comparable to sophisticated cache eviction algorithms like ARC. By implementing a prototype of an NDN software router with Filter, we validate that the NDN router with Filter improves packet forwarding rate compared to that with a sophisticated cache eviction algorithm like ARC and even that with a simple one like FIFO.