Abstract: A state monitoring device of a railcar includes: a monitoring sensor configured to detect state information pieces of a machine part of a bogie; and a wireless transmission unit configured to wirelessly transmit signals at a transmission interval, the signals containing the state information pieces detected by the monitoring sensor. When it is determined that a monitored value based on the state information piece is not more than a threshold, the wireless transmission unit wirelessly transmits the signals at the transmission interval that is a predetermined initial interval. When it is determined that the monitored value has exceeded the threshold, the wireless transmission unit wirelessly transmits the signals at the transmission interval that is a narrow interval narrower than the initial interval.