A voice signal level changing device, comprising: a talk session device; and an adaptor, wherein the adaptor includes a voice input terminal, a voice output terminal and a signal-level adjusting circuit; wherein the adaptor is directly connected to a receiver terminal of a telephone for inputting/outputting a voice signal into/from the telephone, the telephone being connected to a telephone line; wherein the signal-level adjusting circuit is adapted to adjust a voice signal level of the voice signal output from the talk session device to a voice signal level adapted to the telephone and to adjust a voice signal level of the voice signal output from the telephone to a voice signal level adapted to the talk session device; and wherein the talk session device includes: a network interface connected to a network, and the network interface being configured for transmitting/receiving voice information to/from a communication destination equipment via the network; a speaker adapted to emit a voice from an input voice signal; a microphone adapted to pick up a voice to be an output voice signal; and a processing portion (101-106, 108-110, 112-115) adapted to supply voice information input from the network interface as voice signal to the speaker and to the adaptor via the signal-level adjusting circuit, to supply voice signal picked up by the microphone to the adaptor via the signal-level adjusting circuit and to the network interface as voice information, and to supply the voice signal input from the adaptor via the signal-level adjusting circuit to the speaker and to the network interface as voice information. a talk session device; and an adaptor, wherein the adaptor includes a voice input terminal, a voice output terminal and a signal-level adjusting circuit; wherein the adaptor is directly connected to a receiver terminal of a telephone for inputting/outputting a voice signal into/from the telephone, the telephone being connected to a telephone line; wherein the signal-level adjusting circuit is adapted to adjust a voice signal level of the voice signal output from the talk session device to a voice signal level adapted to the telephone and to adjust a voice signal level of the voice signal output from the telephone to a voice signal level adapted to the talk session device; and wherein the talk session device includes: a network interface connected to a network, and the network interface being configured for transmitting/receiving voice information to/from a communication destination equipment via the network; a speaker adapted to emit a voice from an input voice signal; a microphone adapted to pick up a voice to be an output voice signal; and a processing portion (101-106, 108-110, 112-115) adapted to supply voice information input from the network interface as voice signal to the speaker and to the adaptor via the signal-level adjusting circuit, to supply voice signal picked up by the microphone to the adaptor via the signal-level adjusting circuit and to the network interface as voice information, and to supply the voice signal input from the adaptor via the signal-level adjusting circuit to the speaker and to the network interface as voice information. a network interface connected to a network, and the network interface being configured for transmitting/receiving voice information to/from a communication destination equipment via the network; a speaker adapted to emit a voice from an input voice signal; a microphone adapted to pick up a voice to be an output voice signal; and a processing portion (101-106, 108-110, 112-115) adapted to supply voice information input from the network interface as voice signal to the speaker and to the adaptor via the signal-level adjusting circuit, to supply voice signal picked up by the microphone to the adaptor via the signal-level adjusting circuit and to the network interface as voice information, and to supply the voice signal input from the adaptor via the signal-level adjusting circuit to the speaker and to the network interface as voice information.