The J-PARC linac beam pulse, formed by a chopper system placed at the MEBT, consists of a series of intermediate pulses with a comb-like structure synchronized with the rf frequency of the rapid cycling synchrotron (RCS). With a general-purpose digitizer, performing signal processing depending on the presence or absence of such an intermediate pulse is challenging. Besides, monitoring the entire macro pulse cannot be achieved on the current digitizer during the beam operation. However, sequentially measuring and monitoring the comb-like beam, stable operation with less beam loss can be achieved even at higher beam intensity in the J-PARC. To this end, a beam monitor digitizer, including a digital signal processing function, that complies with the MicroTCA.4 (Micro Telecommunications Computing Architecture.4) standard has been developed. We present the architecture and implementation of the beam monitor digitizer within this paper.