On November 16, the girl (10) had gone to the fields near her house in Chaubeypur area to relieve herself when the boy allegedly raped her, police said today.
The boy was held today and sent to a government-run juvenile home in Ramnagar, Station Officer (Chaubeypur) Shamsher Bahadur Singh said.
An FIR has been lodged on the complaint of the girl's father yesterday, he added.
Superintendent of Police (Rural) A K Pandey said they were verifying the veracity of the complaint.