We fabricated junctions with a single PORPI monolayer, where PORPI is a polyimide (PI) Langmuir-Blodgett (LB) film containing a tetraphenylporphyrin (POR) moiety. We then investigated the electron transport properties of the junctions from current-voltage (I-V) and inelastic electron tunnelling spectroscopy measurements. PI LB films without POR were used as tunnelling barriers. Many peaks were seen in junctions with a PORPI monolayer at low voltages, due to the excitation of vibrational modes of the PORPI molecules, whereas no peak was observed in junctions without PORPI monolayers. One large peak was seen at a voltage around 1.9 V, due to the excitation of electron transitions in PORPI molecules, whereas negative electrical resistance was not observed.