The building sanitary plumbing system may be one of the transmission routes for the virus. The trap without water seal can allow the aerosolized virus to enter living rooms from the drainage system; however, no experimental evidence has been presented yet. There is no risk of the virus spreading through the water supply system because the drinking water disinfection is sufficiently effective. The virus in wastewater maybe an early-warning before an epidemic develops. A small scale sanitary plumbing system was built with two simulated toilets on the same floor. This study presents evidence by experiment that virus can be aerosolized and transported on airflow within sanitary plumbing systems and enter buildings via empty traps. Typical sanitary plumbing system airflows were sufficient to carry aerosolized virus between bathrooms due to pressure differences. Traps without water seal allowed virus to enter the negative pressure chamber, encouraging cross-transmission. All test cases of no trap water seal were found to be contaminated with SF6 tracer gas. In hospitals, the infection risk via sanitary plumbing systems increases. Basically, on the premise that the trap water seal is maintained, the exhaust fan should be left running regardless of whether the toilet is used or not, and the door must be always closed.