DECIM v2 is a stream cipher proposed by Berbain et al., and it uses the compression function ABSG which guarantees the security of DECIM v2. It also uses a buffer to ensure that the keystream of a constant length is generated each time. In the conventional works, the candidates of ABSG input sequence is reduced by referring to its output one. But in DECIM v2, there is no report that can recover the exact value of input, because of the characteristic of the buffer. In this paper, we propose a new technique to recover the exact value of the bits input to ABSG. Our technique can recover about 8.33% of input sequence of ABSG. We assume that we can get the timing such that the ABSG output is dropped. This assumption enables us to correlate the timing of the ABSG input and that of the buffer output. Observing the buffer output, we can recover the ABSG input.