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Matrix-assisted laser desorption/ionization (MALDI) using a mid-infrared (MIR) laser is a promising technique for the study of biomolecules. We achieved the measurement of an insoluble protein under strong denaturing conditions using two lasers, a UV laser and a free electron laser (FEL). The FEL is a powerful tool for the IR-MALDI; however, it is expensive and difficult to operate. We developed a MIR-MALDI time-of-flight mass spectrometer combined with a tunable MIR nanosecond pulsed laser, which was developed by the Institute of Physical and Chemical Research (RIKEN; Wako, Japan) and Kawasaki Heavy Industries, Ltd. (Tokyo, Japan). We evaluated the advantages of MIR-MALDI using this MIR nanosecond pulsed laser with a urea matrix and compared the results with that from previous study using the FEL. The molecular mass of insulin with urea was obtained at wavelength between 5.8 and 6.2 mum, which corresponds to the >C=O stretching vibration mode. In particular, a high SNR was observed at a wavelength of 5.9 mum. This technique produced a better SNR than that of a previous study that used the FEL.