A silicon physically unclonable function (PUF) is considered to be one of the key security system solutions for local devices in an era in which the internet is pervasive. Among many proposals, a PUF using ring oscillators (RO-PUF) has the advantage of easy application to FPGA. In the conventional RO-PUF, frequency difference between two ROs is used as one bit of ID. Thus, in order to obtain an ID of long bit length, the corresponding number of RO pairs are required and consequently power consumption is large, leading to difficulty in implementing RO-PUF in local devices. Here, we provide a RO-PUF using the initial waveform of the ROs. Because a waveform constitutes a part of the ID, the number of ROs is greatly reduced and the time needed to generate the ID is finished in a couple of system clocks. We also propose a solution to a change of PUF performance attributable to temperature or voltage change.