To provide a microstrip line/slot line converter which can be downsized while reducing its cost.

The microstrip line/slot line converter includes: a microstrip line 1 having a signal line 5 formed on the surface of a dielectric board 4 and a ground metal pattern 6 formed on a board reverse surface; a first slot line 2 which is connected to the microstrip line 2 and formed between one end surface of a surface metal pattern 7 formed on one side of the board surface and an end surface of a reverse surface metal pattern 8 formed on the other side of the board reverse surface; and a second slot line 3 which is connected to connection parts of the microstrip line 1 and the microstrip line 2 and is formed between the other end surface of the surface metal pattern 7 and an end surface of the ground metal pattern 6. The microstrip line 1 and the first slot line 2 are main lines and the second slot line 3 is a stub.