A detection device includes a substrate on which a connector connected to a transmission line for microwaves, a detection circuit configured to convert the microwaves inputted from the transmission line via the connector to a detection value indicating power of the microwaves, and an output port configured to output the detection value obtained by the detection circuit are disposed. The detection device further includes a housing that has a first opening and a second opening and accommodates the substrate in a state where the connector is inserted into the first opening and the output port is inserted into the second opening. The detection device further includes a first sealing member provided at the first opening of the housing to seal a periphery of the connector; and a second sealing member provided at the second opening of the housing to seal a periphery of the output port.