Han-Jin Kweon had given Kashiwa the lead on the stroke of half-time before Koji Morisaki levelled seven minutes after the break.
A draw appeared to be the most likely outcome but with seconds left Masushima pounced for victory.
Vegalta Sendai took full advantage with a 3-2 home win over Urawa Red Diamonds.
Shingo Akamine put Sendai ahead after just two minutes, a lead that was doubled by Rodrigues Wilson after 62 minutes.
Tomoaki Makino halved the deficit just two minutes later but Wilson restored the two-goal cushion 11 minutes from time.
Marcio Richardes ensured a nervous finish after striking in the 83rd minute but the hosts hung on to move level on points at the top of the table.
Shimizu S-Pulse drew 1-1 with Vissel Kobe. Takahito Soma put the visitors ahead after 13 minutes before Taisuke Muramatsu levelled for S-Pulse on half-time.
Nagoya Grampus Eight won 3-1 at Sagan Tosu. Kensuke Nagai put Grampus Eight ahead right on the break with Cordoba Danilson doubling the lead (67).
Ryunosuke Noda and Marcus Tanaka shared last-minute goals for Sagan and Grampus Eight respectively.
Cerezo Osaka and FC Tokyo shared a 1-1 draw.
Yusuke Maruhashi (50) put the hosts ahead before Tokyo equalised through Nemanja Vucicevic (74). Aria Jasuru Hasegawa was sent off for Tokyo in injury time.
Bottom side Consadole Sapporo had a 0-0 draw with Kashima Antlers. The basement team ended the game with 10 men after Kazuki Kushibiki was sent off with nine minutes to go.
Gamba Osaka won 3-2 at Kawasaki Frontale. Akihiro Ienaga put Kawasaki ahead (21) with the remaining four goals coming in the second half.
Jumpei Kusukami levelled (58) with Hiroki Fujiharu restoring the lead six minutes later.
Kengo Nakamura equalised again after 72 minutes but just four minutes later Ienaga scored his second and what turned out to be the winning goal.
Omiya Ardija held Albirex Niigata to a 1-1 draw. Atomu Tanaka put Niigata ahead (38) with Takuya Aoki equalising a minute from the break.
Yokohama F. Marinos enjoyed an emphatic 4-0 victory over Jubilo Iwata with goals from Marcos Marquinhos (14), Shingo Hyodo (35), Shunsuke Nakamura (71) and Kosuke Nakamachi (84).