CJam, 46 bytes
li:A:B;{A)S*"/ "BA-*'_2A*Bs,+*NA(:A}g"\\/"B*SB
Port of my Python answer
Explanation:
li take input as integer
:A:B; assign that to the variables A and B
{ start of while A loop
A)S* push A-1 spaces
"/ "BA-* push B-A strings of "/ "
'_2A*Bs,+* push 2A + the number of digits _
N push a newline
A(:A decrement A
}g end of while loop
"\\/"B* push B \/
SB push B after a space