A single-cylinder or multi-cylinder four-stroke engine unit comprising: a combustion chamber ; an intake passage member through which air is introduced into the combustion chamber ; and a throttle valve provided in an intermediate portion of the intake passage member , the combustion chamber , the intake passage member , and the throttle valve being provided for each cylinder , pressure in a downstream intake passage portion of the intake passage member that is downstream of the throttle valve varying in a pressure-variation manner such that: a smaller depression having a smaller difference from atmospheric pressure and a larger depression having a larger difference from atmospheric pressure are created in each cycle constituted by four strokes; and the creation of the smaller and larger depressions is performed in each four-stroke cycle, the engine unit further comprising: a canister connected to a fuel tank and accommodating therein an adsorbent configured to adsorb fuel vapor contained in incoming air from the fuel tank ; a communication passage member configured to establish communication between an inside of the canister and the downstream intake passage portion for each cylinder ; a valve capable of being in an open state in which the valve allows communication of air between an inside of the canister and the intake passage member through the communication passage member , characterised in that an opening degree of the valve in the open state being adjustable; and a controller configured to control the opening degree of the valve , whereby the controller is configured to control the valve in such a manner that after the controller keeps the opening degree of the valve in the open state at a predetermined level over a plurality of cycles, the controller changes the opening degree of the valve in the open state, on a basis of a one-cycle-based manner of pressure variation in the downstream intake passage portion , said pressure variation included in the pressure-variation manner in which the creation of the smaller and larger depressions is performed in each four-stroke cycle. a canister connected to a fuel tank and accommodating therein an adsorbent configured to adsorb fuel vapor contained in incoming air from the fuel tank ; a communication passage member configured to establish communication between an inside of the canister and the downstream intake passage portion for each cylinder ; a valve capable of being in an open state in which the valve allows communication of air between an inside of the canister and the intake passage member through the communication passage member , characterised in that an opening degree of the valve in the open state being adjustable; and a controller configured to control the opening degree of the valve , whereby the controller is configured to control the valve in such a manner that after the controller keeps the opening degree of the valve in the open state at a predetermined level over a plurality of cycles, the controller changes the opening degree of the valve in the open state, on a basis of a one-cycle-based manner of pressure variation in the downstream intake passage portion , said pressure variation included in the pressure-variation manner in which the creation of the smaller and larger depressions is performed in each four-stroke cycle. <EOS>