Quantum annealing (QA) is a way to solve combina-tional optimization problems. Kerr nonlinear parametric oscillators (KPOs) are promising devises to implement QA. To solve the combinational optimization problems with QA using the KPOs, it is necessary to know the pho-ton number of the KPOs. Here, we propose a feasible scheme to estimate the number of photons of the KPO. We consider to couple an ancillary qubit to the KPO, and show that spectroscopic measurements on the qubit pro-vide the information of the photon number of the KPO.