Do you mean the rusty sword? I am unaware of a broken sword but there is a rusty sword in Tokyo.
Leave the schoolyard and go east until you find Subaru. Talk to Subaru. When presented with an option, choose, “Yes.”
Go to the southwest side until you get to Kamishiro’s house, then enter the house if you want. Go upstairs to the second floor. You can save and recover some HP if you need to.
Wait for Kamishiro to join you, then go to the upper left side, all the way to the right, and walk through the open wall area. Enter the mansion and go upstairs to the second floor. Look for the treasure chest where you will find the rusty sword.